Exercise Guidelines - Engines 4 Ed
Case-Control and Cohort Studies - FAQs
This FAQ will assist you as you complete the tasks related to Case-control and Cohort studies.
|Case-Control Studies |
|What is a Case-control study and |In a case control study there are typically people who have become sick (or who have some |
|how is it typically used in |disease) and you’re trying to learn more about what caused them to get sick/diseased. |
|epidemiology? |A case-control study compares people who have the disease (cases) to people who don’t (controls) |
| |to find which exposure variable(s) are different between the two groups. If a higher percentage |
| |of cases were exposed to a certain variable (e.g., a food, an environment, a product) than |
| |controls were exposed to, it gives an indication that the variable may be related to the illness.|
| |It’s important to note though, that the results of a case control study won’t tell you the cause |
| |of a disease. The results can only show you the association between the disease and an exposure |
| |variable, or the odds that someone who is sick or diseased was exposed to the variable. The |
| |results help guide decisions of where further investigation should be directed. |
|What is a case? |A case is someone who is sick (has the disease, which is sometimes referred to as the “outcome”).|
| |Other criteria may also be used to define a case (e.g., the person must have become ill within a |
| |certain time period or been present at an event). See Developing Outbreak Case Definitions for |
| |more information on defining cases. |
|What is a control? |A control is someone who looks similar to a case except that they are not sick (or diseased). |
| |Once you have determined who your cases are, you have to select a comparison group of controls. |
| |The controls are people you find that are similar to cases on some important measures (e.g., same|
| |age, gender, income level, or lives in the same city) but DO NOT have the sickness/disease. If a |
| |person has the sickness/disease they cannot be included as a control even if they don’t fit other|
| |criteria of the case definition. Controls must be similar to cases on some other measures so more|
| |accurate comparisons can be made between the two groups. If the controls aren’t similar to cases |
| |on important measures confounding variables may be introduced that can influence the results of |
| |the study. See Matching Case-Controls FAQ for more information on matching controls to cases. |
|What are the steps involved in a |A case-control study identifies a group of people with a disease of interest and compares it to a|
|Case-control study? |group of people without the disease to determine what exposures or risk factors may have |
| |contributed to acquiring the disease. Here are the steps in completing a case control study: |
| |Define a case (those with the disease) and gather all of the known cases you’re going to include |
| |in your study. |
| |Select controls that are similar to your cases, except that they don’t have the disease (match |
| |them for age, gender, income, employment, etc.). |
| |Identify the potential exposure(s) that you think may be associated with the disease or illness |
| |and find out who has been exposed to the different variables through administration of a |
| |questionnaire. |
| |Calculate the OR for each exposure variable you are interested in studying using a 2x2 table (see|
| |the next question for more details). |
|What is a 2X2 table and how does |Here is a common 2X2 table use in epidemiology, also called a 2X2 contingency table: |
|it help me calculate an Odds |Disease No Disease Total |
|Ratio? |Exposed A B A+B |
| |Not exposed C D C+D |
| |Total A+C B+D A+B+C+D |
| | |
| |Note: Depending on the design of the study, the 2x2 table can be set-up many different ways. |
| |You can calculate the OR from the 2X2 table values A, B, C and D using the formula below. |
| |(Important note: in the formula below, the middle column shows you the detailed formula while the|
| |right column shows you the simplified formula that is created by using basic algebra.) |
| |OR = A/(A+C) A |
| |Odds of exposure in disease group C/(A+C) C |
| |______________________ = ________ = _____ |
| |B/(B+D) B |
| |Odds of exposure in no disease group D/(B+D) D |
|What does the term “odds” mean? |The probability that an event will occur is a fraction based on the number of times you expect to|
|How is it different from |see that event divided by the total number of trials. The odds of an event occurring is the |
|probability? |probability that it will occur divided by the probability that it will not occur. Probabilities |
| |always range between 0 and 1, while odds take on different values and can be expressed in |
| |different ways (e.g., 7:2 or ”7 to 2”). |
| |Examples – the probability of rolling a 1 on a six-sided dice is 1/6. The probability of not |
| |rolling a 1 is 5/6. The ODDS of rolling a 1, however, is 1/5, or 1:5, or simply .2. In the above |
| |2x2 table the probability of a person with the disease being exposed to a variable is A/(A+C). |
| |The probability of a person with the disease not being exposed to the variable is C/(A+C). So, |
| |the odds of exposure in the disease group is (A/(A+C)) / (C/(A+C)). |
|What is an Odds Ratio (OR) and |The OR tells you the ratio of the odds that people in the Disease group were exposed to a |
|what does it tell me about |variable vs. the odds that someone in the No Disease group was exposed to the variable. |
|exposure to a certain variable | |
|and the likelihood of becoming | |
|sick/diseased? | |
| |Example |
| |Odds in disease group A/C 2.1 |
| |___________________ ___ = ___ = 1.05 |
| |Odds in no disease group B/D 2.0 |
| | |
| |In the above example, the odds of exposure in the Disease group is similar to the odds of |
| |exposure in the no disease group so the OR is very close to 1. This means that the people in the |
| |Disease group were just as likely to have the exposure as those in the No Disease group. This can|
| |be interpreted to mean that having the disease is not associated with the exposure because people|
| |who had the disease and people who don’t have the disease have the same odds of exposure to the |
| |variable in question. |
| |If the OR is greater than 1 (e.g., 8), then you may conclude that there is an association between|
| |being exposed to the variable and having the disease. The Disease group would have 8 fold greater|
| |odds of exposure than the No Disease group. If the OR is less than 1, you may conclude that there|
| |is an association between not being exposed to a variable and having the disease (e.g., not being|
| |exposed to vitamin C and developing scurvy). The further from 1 an OR is the greater the |
| |association is. Note: You’ll notice in the above formula that the numerator would become the |
| |denominator by changing what is classified as a disease and not a disease, with resultant changes|
| |in the OR. Therefore, an OR less than one has the same associative strength as its reciprocal OR |
| |(e.g., an OR of .125 or 1/8 has the same associative strength as an OR of 8 or 8/1). |
|What is one example of a |There was an outbreak of Giardiasis (symptoms: malodorous diarrhea, nausea, fatigue and weight |
|Case-control study? |loss) among workers at a local hotel. Many hotel employees called in sick and hotel management |
| |began an immediate investigation to try to determine the source of the outbreak. They did lab |
| |tests on all of the employees and gathered all of the cases (those who were diagnosed with |
| |Giardiasis). They then matched the cases with controls from the staff. They surveyed both the |
| |cases and controls and calculated the Odds Ratios for the following exposures: |
| |Those who swam in the pool (OR=1.1) |
| |Those whose kids went to the onsite daycare (OR=5.8) |
| |Those who ate at the hotel restaurant (OR=1.2) |
| |Those who drank the hotel water tap water (OR=1.0) |
| |In the case of the tap water, an OR=1 means that those who were sick had the same odds of |
| |drinking the hotel tap water as those who weren’t sick, which means the tap water isn’t likely |
| |problem. Most of the ORs for the other variables were all close to 1, meaning the likelihood that|
| |the source of infection is from those sources is low. The hotel daycare, however, had an OR of |
| |5.8, meaning that those who were sick had nearly 6 times greater odds of having kids who went to |
| |the onsite daycare. It is important to keep in mind that calculating an OR does not prove |
| |causation, and further investigation would still be needed to verify the source of infection |
| |(e.g., environmental testing at the daycare). |
|Cohort Studies |
|What is a cohort study? |A cohort study involves observation of a group of people over time to measure their exposure to |
| |different variables and see how it relates to their outcomes. With a cohort study, you typically |
| |have a more defined population than in a case-control study and know who has been exposed to |
| |certain variables and you’re looking to see what happens to this group over time in comparison to|
| |what happens to the group who has not been exposed. |
| |In other words, something potentially bad happens (exposure) and you anticipate it may cause some|
| |illness or disease, so you follow the exposed group and compare them to a similar group that |
| |wasn’t exposed. You look at what happens to each group over time and make a comparison that will |
| |tell you the risk of getting a specific illness or disease in relation to the exposure. |
| |Cohort studies can be prospective (following a group for a period of time to look at their |
| |exposure and outcomes) or retrospective (gathering data from things such as historical records or|
| |surveys to determine whether a person was exposed to the variable or not). |
| |In a cohort study you begin by looking at people who have been exposed to a variable and then |
| |investigate whether they develop the disease whereas in a case-control study you begin by looking|
| |at people who have a certain disease and investigate what variables they have been exposed to. |
|What is a “cohort” or cohort |The cohort is a group of people who share a common characteristic or who all experience a |
|group? |particular event at a given time. |
| |Examples - A classic “cohort” is everyone who was born in a certain timeframe (say, between 1980 |
| |and 1985). Other possible cohorts include everyone who attended an event (e.g., wedding, party, a|
| |rock concert), a group or co-workers who worked in the same facility, or everyone who purchased a|
| |certain product. |
|What two groups do you compare in|A cohort study identifies a group of people who have been exposed to a variable (a cohort) and |
|a cohort study? |compares them to a group of people who have not been exposed to the variable to determine what |
| |illness(es) or disease(s) exposure to the variable may be associated with. The comparison group |
| |(unexposed) is frequently matched to the cohort group (exposed) as much as possible (excluding |
| |exposure) to minimize the influence of confounding variables. |
|In a cohort study, how do you | At a high level, here are the steps in completing a cohort study: |
|compare the two groups? |Define the cohort. |
| |Define the exposure(s) of interest and the outcome (disease) you are trying to study. |
| |Decide how you will evaluate the exposure and outcome (e.g., surveys, lab tests, medical records,|
| |exams). |
| |Set-up a 2X2 table for each exposure variable you are interested in studying and calculate the |
| |Relative Risk (RR) for each variable (see the next question for more details). |
|What is a 2X2 table and how does |As in the case-control study, here is a common 2X2 table use in epidemiology (also called a 2X2 |
|it help me calculate Relative |contingency table): |
|Risk (RR)? |Disease No Disease Total |
| |Exposed A B A+B |
| |Not exposed C D C+D |
| |Total A+C B+D A+B+C+D |
| | |
| |You can calculate the RR from the 2X2 table values A, B, C and D using the formula below. |
| |RR = A |
| |Risk of disease in exposed group (A+B) |
| |______________________ = ________ |
| |C |
| |Risk of disease in unexposed group (C+D) |
|What is Relative Risk and what |Relative risk tells you the risk of an outcome (e.g., disease) occurring in those exposed to a |
|does it tell me about exposure to|certain variable versus the risk of getting the disease in those who weren’t exposed to the |
|a certain variable? |variable. |
| |Example - If the probability of getting sick with a certain illness after eating contaminated |
| |spinach was 1 in 200 (.005) and the probability of getting the same illness for other reasons was|
| |only 1 in 1000 (.001) for those who didn’t eat the spinach, then the RR = 5. |
| |RR = 1 |
| |Risk of disease in exposed grp 200 .005 |
| |______________________ = ________ = ______ = 5 |
| |1 |
| |Risk of disease in unexposed grp 1000 .001 |
| |This would be interpreted to mean that you were five times more likely to get sick if you ate the|
| |contaminated spinach than if you didn’t eat it. |
| |But what if the RR was 1.2? |
| |A RR pretty close to 1 means that the risk of disease or illness in the exposed group was about |
| |the same as those in the unexposed group. This can be interpreted to mean that the exposure you |
| |are studying is likely not associated with getting the disease because it appears that people who|
| |were exposed and unexposed became diseased or ill at the same rate. |
| |Note: As with Odds Ratios, RRs greater than 1 are equivalent to their reciprocal RR less than 1. |
| |This, again, is related to how “exposed” can be conceptualized in different ways, changing the |
| |groups from the numerator to the denominator. |
|What is one example of a cohort |There are many different types of cohort studies, but here are the highlights of one cohort study|
|study? |that looked at samples of residents of Framingham, Massachusetts: |
| |Began in 1948, when researchers sought to determine which biological and environmental factors |
| |contributed to death and illness from heart disease. |
| |Studied healthy male and female residents between 30 and 60 years old (had to be free of heart |
| |disease). |
| |Every 2-4 years participants are given extensive exams and surveys to track identified risk |
| |factors (e.g., obesity, level of physical activity) and various heart disease conditions. The |
| |study continues today with remaining members of the original cohort. |
| |Study evaluates several different hypotheses (e.g., increased physical activity is associated |
| |with a decrease in the development of heart disease). |
| |The cohort is determined by separating out individuals into two groups for each risk factor or |
| |exposure under investigation (e.g., level of physical activity). In this example, the exposure |
| |group(s) would be based on the amount of physical activity (low or moderate). |
| |So, those with a low level of physical activity (sedentary) are compared to those with a moderate|
| |level of physical activity to determine what the risk is for heart disease between the two |
| |groups. If the researchers found the RR in this example to be 2.3, then they could say that it |
| |was found that those that had a low level of physical activity were 2.3 times more likely to |
| |develop heart disease than those with a moderate high level of physical activity. |
| |For more information about this study, go to: |
|General Questions |
|What are some ways I can |You should first start with the question you want to ask. |
|determine if I should run a | |
|case-control or a cohort study? |If you want to know the risk of developing a disease based on the exposure, then you would do a |
| |cohort study. |
| |If you know the disease or outcome and you want to know what exposures might be associated with |
| |it, then you would do a case-control study. |
| |If you have a rare exposure with multiple possible outcomes, then you would run a cohort study. |
| |If you have a rare disease, then you would run a case-control study. |
| |If other factors are equal and maintaining low costs is a concern, then you would run a |
| |case-control study. |
|What’s the difference between a |In some ways a retrospective cohort study can look much like a case-control study. The |
|Case-Control Study and a |investigation in both types of studies can begin when a researcher learns that people became ill |
|retrospective Cohort Study? |after an event and then begin to look at exposures. There are, however, differences that separate|
| |the two types of studies. If the cohort were very large, many people in the cohort were |
| |unavailable, not very many people became ill, or costs were an issue you might do a case-control |
| |study rather than a cohort study. As in all case-control studies, you would classify cases by the|
| |presence of illness (using a case definition). You would then randomly select controls who fit |
| |the matching criteria, in this case the matching criteria would be that they were a part of the |
| |cohort (e.g., attended an event). The study would then look for what people in both groups were |
| |exposed to. |
| |Beginning with the same information, that some members of a cohort became ill, a retrospective |
| |cohort study could also be done. If the cohort weren’t very large by the time you were done |
| |selecting enough controls to give a case-control study enough power, nearly the entire cohort |
| |might be contacted. Because the cohort is of a finite size, the cost of doing a retrospective |
| |cohort study would be similar to a case-control study. The difference is that you would attempt |
| |to contact all, or many of the people in the cohort and group them based on exposure (e.g., |
| |people who drank the punch vs. those who didn’t). Even though the study began because some people|
| |were ill, those people would be grouped based on their exposures and NOT based on disease status.|
| |The study would group based on exposure, and then researchers would look to see whether or not |
| |people became ill (outcome). |
| |Note: When conducting a cohort study multiple exposure variables may be researched, with each |
| |exposure variable compared to multiple outcomes. The added cost of doing so can be minimal (e.g.,|
| |it doesn’t take much more time to add a few questions regarding different exposures to a |
| |questionnaire) and the potential of additional results justify the added cost. |
|What is the difference between |In many instances, relative risk is used in cohort studies, and odds ratio is used in |
|relative risk and odds ratios? |case-control studies. Relative risk tells you the risk of an outcome (or disease) in those who |
| |have an exposure vs. those who do not have the exposure. The odds ratio tells you the odds of |
| |having an exposure in those with the outcome of interest (or disease) vs. those who do not have |
| |the disease. |
| |For some examples comparing Odds Ratios and Relative Risk, see this resource - |
| | |
|Why can’t calculating Odds Ratios|The only way to prove causation is through a controlled experiment, where a scientist randomly |
|or Relative Risk prove causation?|assign subjects to two groups, alter a variable one group experiences (the experimental group), |
| |and then measures whether an observable result appears different in the experimental group versus|
| |a control group. This is how drug trials and other research studies work. In epidemiologic |
| |research, it's often not ethically possible to prove causation in this manner. For example, it is|
| |not ethical to ask half your subjects to eat hamburgers tainted with E. coli to see if they |
| |became ill in order to prove causation. |
| |Although researchers cannot always prove causation, there are other data analysis techniques that|
| |let researchers know how much they can trust the results of other statistics, such as Odds Ratios|
| |and Relative Risks. |
|What are the advantages and |See the table below. |
|disadvantages of the Case-Control| |
|and Cohort Studies? | |
|Advantages and Disadvantages of Case-Control and Cohort Studies |
|Case-Control |Cohort |
|Advantages: |Advantages: |
| | |
|Less expensive than cohort |Can measure the risk of developing a disease based on exposure |
|Can look at multiple exposures |Good for rare exposures because the focus of a cohort study is |
|Helpful if you have a rare disease |to track the exposure over time |
|Good to study diseases that have a longer latency period between |Can look at multiple outcomes (disease) of the same exposure |
|exposure and symptoms of disease |Minimizes recall bias |
|Often can be done very quickly | |
| | |
| | |
|Disadvantages: |Disadvantages: |
| | |
|Assess single outcome (disease) only |Expensive, takes a lot of time and money |
|Inefficient for rare exposures. It is like trying to find a needle |Not suited for studying rare diseases because it requires so |
|in a haystack. |many people. (e.g., if a disease typically occurred in 1 out of|
|Can’t measure the risk of developing a disease based on exposure, |50,000 people, the study would have to look at 1 million people|
|only the odds that someone with the disease was exposed to different|to find just 20 people with the disease.) |
|variables. |May require large sample depending on how frequent the outcomes|
|Typically they’re retrospective so there may be recall bias (people |are (similar to above, e.g., occurrence in 1 out of 100 people |
|may forget what happened at the time of the survey or interview, or |still requires a lot of people) and the size of the cohort. |
|those with the disease may have better memory of their exposures |Takes a lot more time than case control because of the |
|than controls). |follow-up period and sometimes you can’t wait |
|It is difficult to be sure that exposure information is accurate. |Because of the long follow-up period, you often lose some of |
|Choice of appropriate control group may be difficult. The controls |your sample along the way. |
|should be representative of the general population while also being |Retrospective cohort studies may have recall bias (see left). |
|matched to controls. Often it is very difficult to satisfy both of |Selection bias, which is a systematic difference in |
|these aims. |characteristics between those who are selected for study and |
| |those who are not. Ideally, the subjects in a study should be |
| |very similar to one another and to the larger population from |
| |which they are drawn. If there are important differences, the |
| |results of the study may not be valid. |
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