We previously questioned descriptive public health studies, noting the they are essential for alerting united state to arising health problems, maintaining track of trends in the population, and generating hypotheses about the causes of disease. Analysis studies provide a simple methodology for testing particular hypotheses. The significance of an analytic study is that groups of subjects are compared in bespeak to calculation the magnitude of association between exposures and also outcomes. This module will build on descriptive epidemiology and on measuring disease frequency and association by pointing out cohort studies and intervention studies (clinical trials). Our discussion of analytic research designs will proceed in module 5 i beg your pardon addresses case-control studies. Pay certain attention come the strengths and also weaknesses of every design. This is crucial for gift able to choose the most suitable design come answer a provided research question. In addition, a firm understanding of the strengths and also weaknesses that each design will facilitate building your skills in vital reading of researches by alerting you to feasible pitfalls and also weaknesses that deserve to undermine the validity the a study.

You are watching: Analytic epidemiologic studies focus on causal associations between exposures and outcomes.

Essential Questions

What room the various strategies because that investigating the reasons or resources of health and wellness outcomes?How perform we select the best method to study a certain health problem?What space the strengths and limitations of different study designs?Learning Objectives

After perfect this section, you will certainly be maybe to:

Explain the function of descriptive public health for defining health problems and establishing hypotheses about the determinants of health and also disease.Explain the utility and the restrictions of situation reports and case series.Describe the design features and the benefits and weakness of every of the following study designs: Cross-sectional studies, eco-friendly studies, retrospective and also prospective cohort studies, instance control studies, and also intervention studiesIdentify the study architecture when analysis an post or abstract.Explain how various study designs deserve to be used to the very same hypothesis to provide different and complementary information.

Overview that Epidemiologic studies

The figure below provides a quick overview that epidemiologic studies. The descriptive researches that have already been discussed are detailed in the optimal part: instance reports and case series, cross-sectional studies, and also ecologic studies. In enhancement to identifying new problems and keeping track of trends in a population, they also generate hypotheses that can be experiment using one of the analysis studies displayed at the bottom.


Note that cohort studies and case-control studies room observational studies, due to the fact that investigators do not clues exposure status. Some exposures are constituent (e.g., one"s genome), some are behaviors and life format choices, and also others are circumstantial, such together social, political, and economic components that affect health. None of these exposures are regulated by the investigators in observational studies; the investigators precise observe, collecting data on this exposures and on a range of health outcomes. In contrast, treatment studies (also called clinical trials or experimental studies) are an ext like a true experiment in that the investigators assign topics to a specific exposure (e.g., one or an ext treatment groups), and also they are complied with forward in time to record health outcomes the interest. Each of this analytic research studies is helpful in details circumstances. Let"s begin by stating cohort studies.

Cohort Studies

Key attributes of Cohort Studies

In cohort research studies investigators enroll people who carry out not yet have the wellness outcomes of interest at the beginning of the monitoring period, and also they evaluate exposure status for a range of possibly relevant exposures. The enrollees are then followed forward over time (i.e., these room longitudinal studies fairly than cross-sectional) and health outcomes space recorded. With this data investigators have the right to sort the topics according to their exposure status for one of the exposures that interest and also compare the incidence of disease among the exposure categories.


For example, in 1948 the Framingham Heart research enrolled a cohort that 5,209 residents of Framingham, MA that were in between the eras of 30-62 and who go not have cardiovascular condition when they to be enrolled. These topics differed from one an additional in plenty of ways: even if it is they smoked, how much they smoked, human body mass index, eat habits, practice habits, sex, family history of heart disease, etc. The researchers assessed these and many other attributes or "exposures" quickly after the subjects had been enrolled and also before any of them had arisen cardiovascular disease. The many "baseline characteristics" to be assessed in a number of ways including questionnaires, physical exams, activities tests, and imaging research studies (e.g., x-rays). They then began "following" the cohort, meaning that they preserved in contact with the topics by phone, mail, or clinic access time in bespeak to identify if and when any kind of of the topics developed any kind of of the "outcomes of interest," such together myocardial infarction (heart attack), angina, congestive love failure, stroke, diabetes and many various other cardiovascular outcomes. They additionally kept monitor of whether their risk components changed.

Over time some subjects eventually began to construct some the the outcomes that interest. Having adhered to the cohort in this fashion, it was eventually feasible to usage the information gathered to evaluate many hypotheses about what qualities were associated with an enhanced risk of heart disease. For example, if one hypothesized that smoking boosted the hazard of love attacks, the topics in the cohort could be sorted based upon their cigarette smoking habits, and one might compare the subset of the cohort that smoked to the subset who had actually never smoked. For each such comparison the one want to make, the cohort can be group according to even if it is they had a provided exposure or not, and one can measure and also compare the frequency the heart attacks (i.e., the accumulation incidence or the incidence rates) in between the groups.

The populace "At Risk"

From the discussion above, it must be obvious that one of the basic requirements of a cohort form study is the none of the subjects have the result of interest at the start of the follow-up period, and time need to pass in order to identify the frequency of arising the outcome.

For example, if one want to to compare the risk of developinguterine cancer between postmenopausal females receiving hormone-replacement therapy and also those no receiving hormones, one would consider details eligibility criteria for the members prior to the begin of the study: 1) they must be female, 2) they have to be post-menopausal, and 3) lock should have a uterus. Amongst post-menopausal females there could be a number who had had a hysterectomy already, maybe for persistent bleeding problems or endometriosis or front uterine cancer. Because these women no longer have actually a uterus, one would desire to exclude, them indigenous the cohort, due to the fact that they room no much longer at hazard of emerging this particular kind of cancer. Similarly, if one want to to compare the danger of developing diabetes amongst nursing residence residents that exercised and those that did not, it would be essential to test the subjects for diabetes at the start of the follow-up period in order come exclude all topics who currently had diabetes and also therefore were not "at risk" of arising diabetes.

Prospective Cohort Studies

Cohort studies have the right to be classified as prospective or retrospective based on when outcomes arisen in relation to the enrollment that the cohort. The Framingham Heart examine is an example of a prospective cohort study. One more well-known prospective cohort research is the Nurses" health and wellness Study. The initial Nurses" health Study (NHS) began in 1976 by enrolling around 121,000 female nurses from throughout the United states who were initially free of well-known cardiovascular disease or cancer. (The Nurses" health Study is currently enrolling the 3rd generation cohort, which includes male and also female nurses).


In a prospective study favor the Nurses wellness Study baseline info is accumulated from all topics in the same means using precisely the same questions and also data collection methods for every subjects. The investigators style the questions and also data repertoire procedures closely in order to acquire accurate information around exposures before disease develops in any of the subjects.

The distinguishing feature the a prospective cohort study is that, at the moment that the investigators start enrolling subjects and collecting baseline exposure information, none of the subjects has developed any kind of of the outcomes of interest.

After baseline details is collected, the participants are complied with "longitudinally," i.e. End a period of time, commonly for years, to identify if and when they become diseased and also whether your exposure status changes. Most studies that this form contact the entrants periodically, perhaps every two years, to update info on exposures and also outcomes. In this way, investigators can at some point use the data to answer countless questions around the associations between exposures ("risk factors") and condition outcomes. Because that example, one NHS examine examined the association between smoking and also breast cancer and also found that there to be no far-ranging association.

Another NHS study examined the association in between obesity and also myocardial infarction. They used reported height and weight to calculation BMII and also categorized ladies into five categories the BMI. The table listed below summarizes your findings through respect to non-fatal myocardial infarction.

BMI# non-fatal MIsPerson-YearsInc. Rate Per 10,000 P-YRate Ratio

Weight, weight change, and coronary heart disease in women. Risk within the "normal" load range. JAMA. 1995 Feb 8;273(6):461-5.

Potential Pitfall: evaluation of prospective cohort studies can take ar only after sufficient time has elapsed so the a sufficient number of subjects have arisen the outcomes that interest. Because the data analysis wake up after some outcomes have actually occurred, part students incorrectly would contact this a retreat study, however this is incorrect. The analysis always occurs after a certain variety of events have actually taken place. The characteristic that distinguishes a study as prospective is that the subjects were enrolled, and baseline data were built up before any type of subjects developed an outcome of interest.

Follow increase in Prospective Cohort Studies

Ideally, investigators desire to have finish follow-up on every subjects, but in large cohort researches that operation for years, there room inevitably civilization who come to be lost to follow up as a result of death, moving, or simply loss of interest in participating. When this occurs, the investigators understand the subject"s exposure condition prior to shedding them, yet not your outcome.

The biggest difficulty with considerable loss to monitor up (LTF) is that it can predisposition the results of the study if the accident are different for among the exposure-outcome categories. This will be shown in the module top top bias.

There is no means to understand if the casualty are different for among the exposure-outcome categories, therefore the only strategy to minimize predisposition from lose to follow up is to save follow increase high (in both prospective cohort studies and clinical trials).

Strategies to preserve Follow Up

Choosing subjects who room motivatedChoosing subjects who are basic to monitor (e.g., registered registered nurses or physicians)Keeping topics interested v newsletters and incentives;Being courteous and also making them feel that they are members of a research study "family"Frequent phone callsMaking questionnaires basic to fill out

Retrospective Cohort Studies

In contrast to prospective studies, retreat studies room conceived after some civilization have currently developed the outcomes the interest. The investigators jump back in time to recognize a cohort of individuals at a point in time prior to they had arisen the outcomes the interest, and they shot to establish their exposure standing at that allude in time. Lock then identify whether the topics subsequently developed the result of interest.

In essence, the investigators jump ago in time to recognize a helpful cohort which to be initially complimentary of an illness and "at risk" of occurring the outcome. They climate use everything records are available to recognize each subject"s exposure standing at the start of the observation period, and they then ascertain what subsequently taken place to the topics in the 2 (or more) exposure groups. Retreat cohort researches are likewise "longitudinal," since they examine health and wellness outcomes over a span of time. The distinction is the in retreat cohort researches some or every one of the situations of condition have already occurred prior to the investigators begin the study. In contrast, exposure details is built up at the start of prospective cohort researches before any subjects have developed any of the outcomes or interest, and the "at risk" period begins after baseline exposure data is accumulated and extends into the future.

Suppose investigators wanted to test the hypothesis that working with the chemicals connected in tires manufacturing rises the hazard of death. Due to the fact that this is a fairly rare exposure, it would certainly be useful to use a unique exposure cohort such together employees the a large tire production factory and also conduct a retrospective cohort study.


The employees that actually functioned with chemicals supplied in the manufacturing process would it is in the exposed group, when clerical workers and also management might constitute the "unexposed" comparison group. Instead of adhering to these topics for decades, it would certainly be much more efficient to use employee health and employment documents over the previous two or three decades as a resource of data. In essence, the investigators room jumping ago in time to recognize the examine cohort in ~ a point in time before the outcome of interest (death) occurred. They can classify them together "exposed" or "unexposed" based upon their employed records, and also they can use a number of sources to identify subsequent result status, such as fatality (e.g., using health and wellness records, next of kin, National death Index, etc.).

Retrospective cohort researches are less expensive and an ext efficient 보다 prospective cohort studies, due to the fact that subjects don"t should be complied with for years. However, the disadvantage is the the high quality of the data is normally inferior to the of a prospective study. In the research of mortality and tire manufacturing chemicals the clerical staff may be much less exposed come the chemicals, but there are likely to be important distinctions in other determinants that affect mortality (confounding factors), such together sex, age, socioeconomic status, education, diet, smoking, alcohol consumption, etc. Employee health and wellness records room unlikely to record this info in sufficient detail to allow the investigators to readjust for distinctions in these other factors. (We will discuss adjusting because that confounding later in the course.)

The differentiating feature that a retrospective cohort research is that the investigators conceive the study and begin identifying and also enrolling subjects after outcomes have currently occurred in few of the subjects.

Strengths and Disadvantages the Cohort Studies

Prospective Cohort Studies

Strengths the Prospective Cohort Studies They deserve to provide far better quality that data on the main exposure and also on confounding variablesSince exposures room assessed prior to outcomes occur, they space less susceptible to bias.

Disadvantages come Prospective Cohort Studies castle are much more expensive and also time consuming.They are not efficient for conditions with long latency.Losses to monitor up can prejudice the measure of association.

Retrospective Cohort Studies

Strengths of retreat Cohort Studies they are useful for rare exposures, e.g., unusual job-related exposuresThey space cheaper and also faster 보다 prospective cohort studiesThey are more efficient for conditions with a lengthy latency period

Limitations of retreat Cohort Studies Exposure data may be inadequate and also there may be poor data on confounding factors, such together smoking, alcohol consumption, exercise, other wellness problems, etc.; old documents were no designed come be offered for future studies

Test Yourself


Selection of topics for Cohort Studies

The choice of topics for a examine is primarily dictated through the study questions and by feasibility.

General Cohorts

For fairly common exposures and health outcomes a general cohort, together as inhabitants of Framingham, MA, can be enrolled. TheFramingham heart Study, which began in 1948, enrolled 5,209 men and also women 30-62 years old. In ~ the time small was known around the determinants of heart disease and stroke, devastating health troubles that had actually steadily boosted in frequency transparent the 20th century. The investigators gathered extensive baseline information with questionnaires, rap tests, and imaging studies. Lock then followed the subjects, and had them return to the research office every two years for a detailed medical history, physics examination, and repeat lab tests. The Framingham study has been enormously successful in giving information about the many important factors of cardiovascular conditions (e.g., hypertension, high cholesterol, smoking, obesity, diabetes, and also physical inactivity). Framingham investigators also collaborate with leading researchers throughout the world on research studies of stroke and also dementia, osteoporosis and also arthritis, nutrition, diabetes, eye diseases, hearing disorders, lung diseases, and genetic trends of usual diseases.

The Nurses" wellness Study and also the black color Women"s wellness Study would also be thought about general cohorts, because they both carry out the possibility to study plenty of exposures and also many wellness outcomes amongst residents v a wide selection of occupations and also circumstances. These studies allow investigators to collection exposure details on many usual exposures (e.g., high blood pressure, smoking, alcohol use, diet, exercise, etc.), and, after adequate follow up time, countless health outcomes deserve to be studied. Once conducting research studies using data from a basic cohort, the recommendation group comes from within the cohort, i.e., an internal comparison group. Because that example, once the Nurses" health Study examined the association between exercise and also heart disease, they closely assessed physical task and computed an in its entirety "MET" score that takes into account the frequency, duration, and also intensity of countless activities. They then sorted castle by MET score, separated the cohort into quintiles (i.e., five an ext or less equal numbers of subjects), and also used the quintile with the lowest MET scores together the referral group against which they contrasted each the the other quintiles. .

Special Cohorts

For rare or unexplained exposures the obvious an option would be a one-of-a-kind cohort that gives a sufficient variety of subjects v the exposure of interest. Examples might include occupational exposures (e.g., asbestos, radiation, and also pesticides), unexplained diets, drug exposures (e.g., pregnant ladies treated v diethylstilbesterol in the 1960s), or rare occasions (e.g., Hurricane Katrina, the bombing of Hiroshima, exposure that responders to the assault on the people Trade facility on 9/11). V special cohorts over there is obviously a focus on a solitary exposure, however many potential health and wellness outcomes have the right to be studied. Another significant difference from general cohorts is that selection of an ideal comparison group can be challenging.

A good example that a special cohort examine is the us Air pressure Health research on the impacts of exposure come dioxin. Throughout the Vietnam War, the U.S. Military sprayedthe herbicide dioxin ("agent orange") end Vietnam to expose opponent supply lines and bases. Airmen to be exposed during spraying flights, when loading the chemical and also while performing maintain on the planes that were used. After the war, combat veterans who had remained in Vietnam complained that a range of health problems. In 1979, the united state Congress directed that an epidemiologic examine be carried out to evaluate adverse wellness effects associated with exposure come dioxin and also other herbicides used during the Vietnam conflict. The examine (informally called the "Ranch Hand Study") enrolled a one-of-a-kind cohort consist of of united state Air pressure pilots who had actually flown objectives to spray dioxin. The comparison group contained Air Force flight crews and also maintenance personnel who served in south-east Asia however had no been connected in herbicide spraying operations. Subjects have been complied with for numerous years, and several analyses have discovered increased all-cause mortality and also cardiovascular mortality in those exposed come dioxan. There was likewise evidence of an association v obesity and possibly diabetes. There were conflicting reports about the association in between dioxan and also cancers.

Selection of a comparison Group

The major difficulty for the Air pressure Health study (AFHS) and also other special cohort studies is an option of an appropriate comparison group. The score of analytic researches is to compare health outcomes in exposed and also unexposed teams that room otherwise as comparable as possible, i.e., having actually the same distributions of all other determinants that might have any kind of association with health outcomes. Us will view that treatment studies with large numbers of topics randomly assigned to two or an ext treatment teams (exposures) have the right to usually attain this so that the groups being contrasted have comparable distributions of age, sex, smoking, physical activity, etc., yet random assignment walk not happen in cohort studies. Expect that a cohort study had actually smokers who were older than the non-smokers. It is well created that the risk of heart condition increases v age, i.e., the is an live independence risk variable for love disease, and also if the smokers are older, lock have an additional risk element that will reason an overestimate of the association between smoking and heart disease. This phenomenon, called confounding, occurs once the exposure groups that are being compared differ in the distribution of other determinants of the outcome of interest. An additional concern is that the exposure teams being compared may different in the quality or accuracy that the data that is being collected, and also this can also bias the results (so-called information bias). Confounding and bias will certainly be questioned later in the course, yet for now, it is necessary to identify the prestige of choosing a comparison group that differs in exposure status but is as comparable as feasible to the exposed team in all other means including:

Other factors that can influence the wellness outcomeThe quality and accuracy of their data

The figure listed below depicts three research studies of cardiovascular disease illustrating the basic approaches to picking a comparison team for a cohort study.


As listed earlier, general cohorts rental an internal compare group, e.g., splitting the cohort right into quintiles of BMI or quintiles of task and utilizing the quintile with the lowest BMI or the lowest task as the referral group. This is the finest comparison team for a general cohort study, since the topics are likely to be comparable in some ways, however they may still differ v respect to potentially confounding factors. For example, nurses who practice regularly may be generally an ext health conscious (e.g., much less likely come smoke; more likely to eat a healthy diet; much more likely to take it vitamins, etc.).

The second technique is to use an external comparison group. A distinct exposure cohort consist of of workers in a rayon factory, to be selected to study the association between disulfide exposure and risk of cardiovascular disease, and also the compare group contained workers in a file mill. This two groups may be similar in age distribution, socioeconomic status, and other factors, yet they may likewise differ v respect to other confounding factors. In addition, file mills have actually their own mix of job-related exposures, which can also affect the likelihood the cardiovascular condition and bias the results.

The 3rd approach is to use the general populace as a comparison group, for example, if make the efforts to determine whether employees in a rayon manufacturing facility had higher mortality rates. This technique is much less costly, and it is periodically used for researches of occupational exposures as soon as it is difficult to find an appropriate internal or outside comparison group. However, using rates of fatality or an illness in the general population has a number of limitations:

General populace data room frequently limited to studies of mortality because accurate rates on details health outcomes might not it is in available.General populace rates incorporate exposed and unexposed individuals.The general population is no really comparable due to the fact that there are numerous confounding variables the cannot be managed for.The general population includes human being who room unable to work due to the fact that of condition or handicap (the "healthy worker effect" i m sorry is questioned in the module ~ above bias).

Test Yourself

Basic evaluation of Cohort examine Data

One that the first steps in the analysis of one epidemiologic study is to generate simple descriptive statistics on each of the teams being compared. This help characterize the research population, and also it likewise alerts you and your readers to any type of differences between the groups with respect to other exposures the might cause confounding.

The illustration listed below is Table 1 native the research by Manson et al. On exercise and prevention the cardiovascular disease. Recall that they calculated each subject"s MET score to estimate their overall task level and then split the cohort into quintiles based on the MET scores.


There space columns because that each that the 5 quintiles in order from the least active to the many active. The rows list countless variables that characterize the subjects and also could likewise be confounders. Note that dichotomous variables space listed an initial and the percent with a provided characteristic is provided for each quintile. For example, 28.2% the quintile 1 were present smokers, and also this decreased steadily to 17.5% in the most active group (quintile 5). Therefore, smoking cigarettes will be a potential confounding factor, since it is a risk variable for cardiovascular disease, and also it differs among the exposure groups. Other possible confounding components in table 1 include background of hypertension, history of diabetes, background of hypercholesterolemia (high blood levels of cholesterol), present use that hormone instead of therapy, usage of multivitamins, and also use the vitamin E supplements.

Continuous variables are noted in the lower fifty percent of Table 1, mirroring the median value for each quintile of activity. Age is a risk element for cardiovascular disease, yet it is unlikely to cause confounding in this specific study, since the mean period is 52.1-52.3 year in all five quintiles. However, several of the other continuous variables do differ across the exposure groups, e.g., human body mass index, alcohol consumption, and also dietary cholesterol. Overall, increasing activity seems to be connected with fads in characteristics connected with a healthy lifestyle. If our score is to understand the independent result of practice on hazard of love disease, climate one must change for as plenty of of these confounding components as feasible in the succeeding analysis. You will certainly learn just how to carry out this later in the course once we comment on confounding more completely.

You learned just how to use R to create descriptive statistics in the introduce module ~ above R, and also you have the tools to create a table like Manson"s Table 1 native a data set. The just other tool that you require is how to generate descriptive statistics in subsets that the data, e.g., the quintiles in the study by Manson et al. Methods for sub-setting room presented top top the following page.

Analyzing Data in Subsets making use of R

The tapply() command

The tapply() function is advantageous for performing functions (e.g., descriptive statistics) on subsets that a data set. In effect this enables you come subset the data by one or much more classifying factors and then performing some duty (e.g., computing the mean and also standard deviation of a given variable) by subset. Note that tapply() is provided for descriptive statistics (e.g., mean, sd, summary) because that continuously dispersed variables. Because that categorical variables you must use the table() function to gain counts the categorical variables and use the prop.table() role to get proportions. The simple structure that the tapply command is:


where is the variable that you desire to analyze, is the variable that you desire to subset by, and also is the role or computation that you want to use to .

For example, mean I have a data set with constant variables Dubow (Dubow Score), DrugExp (Drug Exposure) and also Ppregwt (Pre-pregnancy weight). My score is to kind the data collection by DrugExp and also then compute the mean and standard deviation that Dubow Scores and also Pre-pregnancy weights because that each classification of DrugExp.

> tapply(Dubow,DrugExp,mean) # Gives means of Dubowitz score by drug exposure> tapply(Dubow,DrugExp,sd) # gives the standard deviations that Dubowitz score by medicine exposure> tapply(Ppregwt,DrugExp,mean) # gives the way of pre-pregnancy weight by drug exposure> tapply(Ppregwt,DrugExp,sd) # offers the traditional deviations the pre-pregnancy weight by drug> tapply(Birthwt,DrugExp,t.test) # gives 95% confidence interval because that exposed and unexposed in one output

An Alternate an approach of Subset Analysis

Getting descriptive statistics by classification can also be completed as follows:

> mean(Birthwt); mean(Birthwt) # method for each exposure group > sd(Birthwt); sd(Birthwt)# typical deviation because that each exposure group > t.test(Birthwt) # 1-sample t-test to gain 95% CI for those exposed come drugs> t.test(Birthwt) # 1-sample t-test to acquire 95% CI because that those unexposed to drugs

Using the double equal sign (==) basically way "only if DrugExp amounts to 1".

Creating a Dichotomous change from a constant Variable

Suppose my data set has a continuously distributed variable dubbed "birthwgt", i beg your pardon is each child"s load in grams in ~ birth, but I wish to create a new variable that categorizes kids as having actually Low Birth weight (lowBW), i.e. Much less than 2500 grams or not. I have the right to do this making use of the ifelse() function, which has the following format:

> ifelse(, , )


> lowBW

If the change birthwt is much less than 2500, then the brand-new variable lowBW will have actually a worth of 1, definition "true"; if not, that will have a worth of 0 an interpretation "false". As soon as this command is executed, you must see the new variable present up in the worldwide environment window at the top right corner of RStudio. Keep in mind that you must reattach your data collection so the the brand-new variable will certainly be recognized.If you want the loBW classification to include those whose weight was specifically 2500 grams, then use > lowBW

Crude procedures of association in a Cohort examine (or treatment Study)

After generating the descriptive statistics because that an epidemiologic study, the following step is come generate approximates for the size of association in between the main exposure of attention (e.g., physical activity level in the Manson study) and also the primary outcome of interest (e.g., breakthrough of cardiovascular disease). As noted above, there might be confounding factors that have the right to distort the estimated measure that association, yet one still begins by generating crude measures of association, i.e., approximates that have not however been adjusted for confounding factors.

Test Yourself

The table listed below shows data native the top part of number 2 indigenous the study by Manson et al.

Table – relative Risk the Coronary occasions According come Quintile team for full Physical Activity

Quantile Group based on Physical Activity













Number the coronary events






Person-years of follow up






Using the data in the table above, a) compute the incidence rate ratio and also the incidence rate distinction for moderate task compared come the least active subjects, and b) create an interpretation of her findings. Complete both parts before comparing your answers to those in ~ the link below.


Intervention studies (Clinical Trials, speculative Studies)

Intervention studies (clinical trials) are similar to prospective cohort research studies in architecture in that topics with or there is no a given exposure are complied with over time to to compare incidence the the result of interest. The crucial difference is that prospective cohort studies space observational, but in clinical trials the investigators assign topics to the exposure groups


While this design is commonly used come evaluate new drugs, it deserve to be used to advice the efficacy of

Diets (e.g., main prevention that cardiovascular condition with a Mediterranean diet)Exercise regimens (e.g., a clinical trial of practice to reduce post-partum depression)New program (e.g., pre-natal treatment in groups of 8-10 ladies versus usual one-on-one pre-natal care)New clinical administration schemes (e.g., a protocol to mitigate post-operative complications)

However, unlike prospective cohort researches in which investigators record exposures that subjects currently have, in clinical trials the investigators assign patients to among the exposure groups being compared. Ideally, this assignment is done with random allocation, an interpretation that each subject has actually an equal possibility of being assigned to any one that the "exposures."

Ethical Considerations

Investigators assign patients to contending treatments in clinical trials, and this raises the inquiry of even if it is it is ethical to do this. Certainly, that is not honest to test all exposures in this fashion. It would certainly be unethical, because that example, to command a clinical attempt on the impacts of smoking, specifically since we understand that the harm caused by smoking much outweighs any kind of potential benefits, such together relaxation or weight control.

On the various other hand, think about a case in i beg your pardon a brand-new drug has been occurred to treat breast cancer. Probably it has actually been uncovered to be efficient in cell cultures and also in pet models, and also perhaps preliminary studies in small groups of human volunteers have presented some proof of effectiveness with minimal side effects. TIn other words, over there is reason to think that it might be a beneficial new treatment, but there is additionally doubt.about efficiency and feasible side effects. Experimentation on a huge scale v a comparison team may present that that is no so efficient or the its side results are unacceptable. This is what is described as equipoise, i.e., the balance between sufficient belief in the potential benefit and safety the one can justify exposing some topics to it and sufficient doubt around its benefit and also safety the one have the right to justify withholding that from some subjects.


It is unethical to conduct a clinical attempt in the lack of equipoise, and if equipoise end to exist during the food of a clinical trial, the trial must be discontinued.

Before study on living humans is conducted, a comprehensive protocol should be it is registered to one Institutional testimonial Board (IRB) because that review and also approval. This is true not just of clinical trials, but likewise all other varieties of human research consisting of case-series, cross-sectional surveys, prospective and retrospective cohort studies, and case-control studies.

"Human Research" is identified as any kind of systematic investigation involving living human beings (including research development, testing and evaluation), design to build or contribute to generalizable knowledge.

Informed Consent

One that the key things the an IRB will take into consideration is even if it is potential subjects have provided informed consent, i beg your pardon is the procedure by which examine participants consent to it is in subjects just after becoming totally informed and also understand all elements of the research including the purpose, risks, type of details to it is in collected, potential benefits, and alternatives to the research. Informed consent should enable people to make a fully informed decision about whether to take part in a study or not based upon their own goals and also values. Informed consent should be obtained before assignment to a treatment group, and also consent can be withdrawn at any type of time throughout the study.

Potential participants have to be fully informed about:

The objective of the studyThe treatment options (including alternatives) and also the potential outcomesThe risks and potential benefit of the studyRandomization, i.e., that the treatment they receive is not their choiceWhat will be forced of lock (questionnaires, visits, samples collected, etc.)Their capacity to withdraw from the research at any type of time without consequence

Types of intervention Studies

Therapeutic vs. Preventive

Clinical trials in individuals deserve to be classified together either therapeutic or preventive, together in these examples:

Therapeutic Trials: brand-new treatments room tested because that the effectiveness in treating disease, e.g.,

Does the medicine herceptin enhance survival in women currently diagnosed through breast cancer?Does treatment through Tamiflu shorten the duration and improve survival in patients with bird flu?

Preventive Trials: healthy or high-risk individuals are experiment to recognize whether a treatment avoids disease, e.g.,

Does the drug tamoxifen prevent breakthrough of chest cancer in females who have actually a high danger of developing breast cancer?How efficient is this year"s influenza vaccine in avoiding the flu?Does a Mediterranean diet minimize the incidence the cardiovascular disease?

Individual Trials vs. Neighborhood Trials

Preventive actions can likewise be allocated top top a community level – so-called neighborhood trials. A classic example is the Newburgh-Kingston Caries Fluoride research which started in 1947. Fluoride was added to the water it is provided of Newburgh, NY, and the incidence of dental caries in Newburgh to be then compared to the incidence in Kingston, NY, which go not obtain fluoride. The trial demonstrated that enhancement of tiny quantities of fluoride come the water supply reduced dental caries by 2 thirds in kids who started drinking fluoridated water within their first two years.

The crucial difference is the in community trials the therapies being studied space allocated not to individuals, however to entire communities.


Phases of separation, personal, instance Therapeutic Trials

When most human being hear referral to a clinical trial, lock think of step 3 trials in which huge numbers of subjects are enrolled and also randomly assigned to one of the therapy groups. However, phase 3 trials of brand-new drugs with potentially harmful side effects are came before by considerable studies in laboratory animals and also by step I and phase 2 trials in human volunteers.

Phase 1 Clinical Trials

If researches in animals suggest efficacy and also safety, a phase 1 trial deserve to be carried out in a tiny group (10-30) of human being volunteers end 2-12 months, mainly to test for safety and also to recognize side effects, but likewise to get some info on reliable dose.

Phase 2 Clinical Trials

Phase 2 clinical trials involve more volunteers than phase 1, and they commonly last about two years. They commonly involve two or more groups receiving different doses that the new drug in bespeak to develop its therapeutic selection of the drug, i.e., doses at which that is effective and also has an agree level of next effects. If results imply efficacy and also safety, a phase 3 trial will certainly be conducted.

Phase 3 Clinical Trials

Phase 3 trials are similar to prospective cohort studies in their design, except that the exposure of interest is a drug or part other treatment that is randomly assigned to the entrants by the investigators. Come facilitate this presentation of phase 3 trials we will emphasis on the first Physicians" health and wellness Study, which began in 1981 in bespeak to check the efficacy the aspirin in major prevention that myocardial infarction. A second goal of the research was to evaluate the efficacy of beta-carotene in preventing cancer, yet this conversation will focus on the aspirin component.

The Physicians" health and wellness Study ~ above Aspirin

As beforehand as the 1950s there to be case collection and little clinical trials saying that aspirin can be helpful in avoiding myocardial infarction (heart attack). However, the palliation in risk showed up to it is in modest, and also the researches were too small to show a statistically far-reaching benefit. Therefore, investigators at Harvard clinical School sought funding for a big phase 3 clinical trial.


In 1981, ~ receiving approval indigenous the Institutional review Board at Harvard clinical School, the investigators mailed invitation letters, consent forms, and enrollment questionnaires to all 261,248 registered male medical professionals inthe US in between 40 and 84 year old. (Phase 1 and also phase 2 trials to be unnecessary, due to the fact that aspirin to be a typically used drug with known dosage range and recognized side effects.)


Questionnaires were went back by 112,528 physicians, yet only 59,285 of those to be willing to get involved in the trial. Of those, 26,062 might not participate since they had one or much more of the exemption criteria:

past myocardial infarction, stroke, or transient ischemic attack;cancer (except non-melanoma skin cancer);current renal or liver disease;peptic ulcer;gout;contraindication come or present use of either aspirin or beta-carotene.

Informed consent was acquired from the 33,223 that were willing and also eligible come participate. Since regular aspirin use has actually the potential to cause gastritis and bleeding problems, these doctors were enrolled in one 18-week run-in phase, throughout which all received active aspirin and also placebo beta-carotene for 18 weeks. Some had unpleasant side effects, others decided not to participate, and some to be excused since they didn"t take it the medications reliably. The staying 22,071 males were climate randomly assigned come one of four treatment groups.


Randomization and also Blinding (Masking)


Randomization is a method of allocating topics in a clinical trial to treatment teams such the every subject has actually an equal opportunity of receiving any type of one that the treatments or interventions. This have the right to be accomplished by any kind of fair an approach that assigns topics in a completely unpredictable fashion. One can use the upper and lower reversal of a coin if there are only two therapy options, but much more commonly a table of random numbers or computer-generated random numbers space used. Various other methods, such together assigning subjects based upon odd or even calendar date, can be "gamed" in a way that biases assignment.

If assignment is truly unpredictable, then there is no prejudice in assignment, and neither the topics nor the investigators can influence assignment. In addition, randomization that a big number of topics tends to result in teams that differ only in treatment and are equivalent with respect to all various other factors and also characteristics that can influence the outcome. Together a result, randomization is the best method for removed confounding.


Blinded (or "masked") studies space those in which the subjects, and also possibly the investigators as well, room unaware the which therapy the subject is receiving, e.g., energetic drug or placebo. Blinding is particularly important in medicine trials when the examine is assessing subjective outcomes, such together relief of pains or anxiety.

It isn"t always feasible to mask the treatments. For example, topics randomly assigned come follow either a specific exercise regimen or proceed their normal level of task cannot be blinded.

Single-blinded: the subjects are unaware of which group they have been assigned to.Double-blinded: neither the subjects nor the investigators are mindful of the treatment assignment until the finish of the trial.

A placebo is an inert substance the same in appearance come the energetic treatment. Its objective is come facilitate blinding by do the groups as similar as possible in the tardy of treatment and also to encourage compliance. In the Physicians" health Study participants were given a blister pack for every month (shown in the picture below) that included white tablets and also red capsules that were taken on alternating days. The white tablets consisted of either 325 mg. Of aspirin or an identical-looking inert substance; the red capsules consisted of either beta-carotene or one inert substance. The usage of monthly blister packs additionally made it less complicated for entrants to keep track of even if it is they had actually taken the exactly pill every day.


It is not constantly ethical to use a placebo. If there is currently a standard treatment or technique of care, it would be unethical to withhold it. A brand-new treatment need to be contrasted to the typical therapy rather than to a placebo.

Example the Placebo use to achieve Blinding:

Glucosamine and chondroitin space naturally developing substances that space structural components of the cartilage that lines ours joints. Health and wellness food stores began selling additional to people as a prevention (or treatment) for osteoarthritis regardless of a lack of evidence of their advantage in humans. Clegg and also colleagues performed a double-blind, randomized clinical trial in 1583 subjects with symptomatic osteoarthritis the the knee. Participants were randomly assigned come one of 5 treatment arms in bespeak to check the efficacy of glucosamine and chondroitin. The major outcome was greater than 20% decrease in complete score top top the WOMAC pain range from baseline to week 24. Some of their results are presented in the table below.

Pain relief >20%Minimal EffectTotal # Subjects
Anti-inflammatory drug

Glucosamine + Chondroitin211106317

Data from Clegg DO, et al.: Glucosamine, chondroitin sulfate, and the twoin combination for pains knee osteoarthritis. N Engl J Med 354:795, 2006.

Perhaps the many remarkable observation is the response in the group treated with the placebo which had a cumulative incidence that >20% ache relief the 60% (188/313 = 0.60 = 60%)! This is an instance of the "placebo effect" in which patients who perceive they are being treated regularly report subjective improvement, even if the treatment has no effect. Placebos do the perception of treatment similar among groups and provide a reference group that takes right into account the placebo effect. Note additionally that the group treated v glucosamine and chondroitin had only a contempt greater response rate of 67%.

Analysis that Clinical attempt Data

The analysis of clinical trial data is very similar to the previously described analysis of data native a cohort study. The very first step is to generate an easy descriptive statistics on each of the groups being compared in order come characterize the study population and alarm you and your reader to any type of differences in between the teams with respect to other exposures the might cause confounding. If large numbers the subjects have actually been randomly assigned to the treatment arms, the groups should be comparable. If over there are much more than young discrepancies, the investigators should review the randomization procedures and also consider adjusting for confounding by other methods.

The table listed below shows just a section of the data from the table of descriptive statistics native the Physicians" health Study ~ above aspirin.

Aspirin (n=11,037)Placebo (n=11,034)
Age (years)53.2 ± 9.553.2 ± 9.5
Systolic BP (mm Hg126.1 ± 11.3126.1 ± 11.1
Diastolic BP (mm Hg)78.8 ± 7.478.8 ± 7.4
History the hypertension (%)13.513.6
History that high cholesterol (%)

Cholesterol level212.1 ± 44.2212.0 ± 45.1
History that diabetes (%)2.32.2

Note the the two teams were remarkably comparable on these and also other characteristics, indicating the randomization had been successful.

After generating the descriptive statistics, the following step is to generate crude estimates for the size of association between the major exposure and also the outcomes the interest.

Test Yourself

after ~ 5 year of monitor up In the Physicians" health Study, an interim evaluation found that among the 11,034 guys assigned come the placebo team there had been 213 non-fatal myocardial infarctions. Among the 11,037 males assigned take it 325 mg. The aspirin every various other day, there had been 126 non-fatal myocardial infarctions.

Summarize these finding in a contingency table and also compute the accumulation incidence in every group, the danger ratio, and also the risk difference. Then translate the threat ratio and also the risk difference. Complete all of these tasks before comparing her answers come the ones listed in the link below.


Strengths and Limitations that Clinical Trials

Large randomized clinical trials can provide strong evidence that the true impact of a therapy or intervention, because they carry out excellent control of confounding, yet they additionally have part limitations:

Strengths of intervention Studies (Clinical Trials)

They provide the best means of minimizing the impact of confoundingThey avoid bias in allocation come exposure groupsLarge randomized clinical trials are the best style for detecting small to moderate effects that may be clinically important


Ethical issues need to be thought about Risks to topics versus potential benefitsDoes equipoise exist? Some concerns cannot it is in answered ethically v a clinical trial.They room usually time consuming and costlyLengthy trials operation the hazard of lose to monitor up (LTF), and also if LTF is different for one of the exposure-outcome categories, the measure up of association will be biased, just just like prospective cohort studies(see the module on Bias).Invariably, some subjects will fail to adhere come the protocol, and non-adherence will cause an underestimated measure up of combination (see below).


Ideally, the investigators desire to compare exposed topics to non-exposed in groups that are similar with respect come confounding factors. The true benefit of a new drug will be underestimate if subjects offered the energetic medication failure to take it it, leading to subjects who were actually no exposed to be blended in through the exposed subjects who to be actually taking the medication. This mixing of the exposure groups dilutes the obvious benefit bring about underestimates the association. The same thing wake up if people in the placebo group begin taking the active medication. This developed in the Physicians" health and wellness Study in which monitor up questionnaires approximated that about 15% that the topics assigned to the aspirin team did not take it, and also a comparable proportion of subjects in the placebo team used aspirin fairly regularly. This would cause an underestimate the the true benefit. In this case, in i beg your pardon the exposure was preventative with an it was observed risk proportion = 0.59, the true risk ratio would have been even smaller. In other words, non-adherence caused a "bias towards the null," an underestimate the the true benefit.


Non-compliance deserve to occur due to side impacts of the treatment, illness, or loss of interest in the study.

How come Promote adherence in a Clinical Trial

Begin through an interested group of participantsMake it easy to participate current a realistic picture of the protocol during informed consentExclude participants v pre-existing problems that do compliance difficultSimplify the protocol as lot as possibleConduct a run-in period if necessaryUse blinding and placebosKeep in touch maintain frequent contact with topics WITHOUT interfering through treatmentProvide incentives (free check-ups, transportation, t-shirts, date of birth cards)

Data Safety and also Monitoring Board

All clinical trials that involve much more than minimal threat are compelled to have actually a Data Safety and also Monitoring board (DSMB), i beg your pardon is an independent board of experts not connected in the examine who periodically testimonial the data in a trial to advice safety, examine conduct, and interim results. They have the right to recommend the the study be continued, modified, or terminated. The DSMB for the Physicians" health and wellness Study recommended the the research be terminated after 5 years since the benefits of taking low-dose aspirin were so clear that continuing to withhold aspirin native the placebo group was not ethically justified. The DSMB felt the equipoise no longer existed.

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Intention-to-Treat evaluation versus Efficacy Analysis

The greatest benefit of big randomized clinical trials is that they provide control that confounding. However, together already noted there deserve to be troubles due to loss to follow up and lack of follow to the protocol. It might be tempting to border the evaluation to topics who completed the study and also who adhered to the research protocol, however this efficacy analysis may no provide strong control the confounding, since subjects have, in essence, self-selected whether they would stay in the study and also adhere come the protocol. For this reason, well-done clinical trials will certainly conduct and report the results of one intention-to-treat analysis in which topics are had in the analysis in the groups to which they were randomly assigned nevertheless of even if it is they adhered to the protocol. We already listed that non-adherence will bias the outcomes toward the null, i.e., underestimate the association if over there is one. However, the intention-to-treat evaluation provides the ideal opportunity to research the combination in the lack of confounding. Plenty of reports will provide the results of the intention-to-treat analysis and the efficacy analysis as well, and they may additionally analyze sub-groups that subjects, but these analyses must use other approaches to minimization the impacts of confounding.