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1 The Young Epidemiology Scholars Program (YES) is supported by The Robert Wood Johnson Foundation and administered by the College Board. Case Control Study Mark A. Kaelin Department of Health Professions Montclair State University Upper Montclair, New Jersey and Manuel Bayona Department of Epidemiology University of North Texas Fort Worth, Texas

2 Contents Lesson Plan Glossary Notes for Teachers Teacher s Narrative Transparencies Worksheet A: Case Control Studies and Odds Ratio Worksheet B: Case Control Studies and Odds Ratio Assessment (Student Version) Assessment (Teacher s Answer Key) Optional In-Class Exercise (Student Version) In-Class Exercise (Teacher s Answer Key) Copyright 2004 by College Entrance Examination Board. All rights reserved. College Board, SAT and the acorn logo are registered trademarks of the College Entrance Examination Board. Other products and services may be trademarks of their respective owners. Visit College Board on the Web: 2

3 Lesson Plan TITLE: SUBJECT AREA: GOAL: Case Control Study Biology, mathematics, statistics, environmental and health sciences To appreciate the value and limitations of the case control study design OBJECTIVES: 1. Introduce students to the principles and methods for designing and interpreting the results of a case control study 3. Apply descriptive and analytical techniques in epidemiology, including the calculation and interpretation of the odds and the odds ratio 4. Understand the differences and the strengths and limitations of case control studies as compared to other epidemiologic research designs 5. Identify the circumstances in which a case control study would be an appropriate study design to test a hypothesis TIME FRAME: Three 60-minute classes, with 2 hours of work outside class and including the optional in-class exercise PREREQUISITE KNOWLEDGE: Basic knowledge of the cohort study design (see cohort study module), the 2 2 table, risk and relative risk MATERIALS NEEDED: Copies of article, Worksheets A and B, and a hand calculator PROCEDURE: The instructional unit has nine different sections: 1. Notes for teachers: Notes with basic concepts and procedures related to case control studies and the calculation, interpretation and use of the odds ratio as a measure of association. 2. Teacher s narrative: The teacher s step-by-step instructions for teaching the module. Includes assigned reading an article assigned for homework after Class 2 (Shands KN, Schmid GP, Dan BB, et al. Toxic-shock syndrome in menstruating women: association with tampon use and Staphylococcus aureus and clinical features in 52 cases. New England Journal of Medicine. 1980;303: ). 3. Transparencies: Transparencies that accompany Classes 1 and Worksheet A: In-class questions that accompany Class Worksheet B: In-class questions that accompany Class 2. 3

4 6. Assessment: Questions can be used as a quiz, homework assignment or in-class exercise to assess students understanding. 7. Assessment key: Suggested answers to assessment questions. 8. In-class exercise. 9. In-class exercise: Suggested answers to in-class exercise. Recommended References Friis RH, Sellers TA. Epidemiology for Public Health Practice. Gaithersburg, MD: Aspen Publishers: Kelsey LJ, et al. Methods in Observational Epidemiology. 2nd ed. Monographs in Epidemiology and Biostatistics. New York: Oxford University Press; Lilienfeld DE, Stolley PD. Foundations of Epidemiology. 3rd ed. New York: Oxford University Press; Mausner JS, Kramer S. Epidemiology An Introductory Text. Philadelphia: WB Saunders; NATIONAL SCIENCE EDUCATION STANDARDS: Science As Inquiry Abilities necessary to do scientific inquiry Understanding of scientific inquiry Science in Personal and Social Perspectives Personal and community health Natural and human-induced hazards Unifying Concepts and Processes Systems, order and organization Evidence, models and measurement National Science Education Standards, Chapter 6, available at: National Standards for School Health Education: Students will comprehend concepts related to health promotion and disease prevention. Students will demonstrate the ability to access valid health information and healthpromoting products and services. Students will analyze the influence of culture, media, technology and other factors on health. 4

5 Students will demonstrate the ability to use goal-setting and decision-making skills to enhance health. The National Standards for School Health Education available at: ericdigests/ed html 5

6 Glossary Case control study An analytical epidemiologic study design in which individuals who have the disease under study, also called cases, are compared to individuals free of disease (controls) regarding past exposures. Exposure differences between cases and controls are helpful to find potential risk or protective factors. The purpose is to determine if there are one or more factors associated with the disease under study. Contingency table (2 x 2 table) A table commonly used to display results of epidemiologic studies to calculate measures of disease frequency and association from dichotomous categorical variables. A typical 2 x 2 table in epidemiologic studies is as follows: Cases Noncases Exposed a b Nonexposed c d A 2 2 table is commonly used to calculate the odds ratio (OR = ((a/c)/(b/d)). In cohort studies but not case-control studies, it can also be used to calculate the relative risk (RR = (a/(a + b)/c/(c + d)). Incidence rate Odds Odds ratio A calculation of disease frequency that measures the probability of developing a disease in a given period of time. It is calculated by dividing the number of new cases by the total number of people susceptible to the disease at the beginning of the study period. As with other rates, the result may be multiplied by a multiple of 10 to obtain a convenient number. In epidemiology, odds of an attribute can be defined as the result of dividing the proportion of individuals with an attribute by the proportion of individuals without the attribute in a population. Odds can be interpreted as the probability of occurrence of an event as compared to the probability of nonoccurrence of an event in a population. A measure of association typically used to quantify the strength of association between a potential risk or protective factor (exposure) and an outcome. The odds ratio (OR) is a measure of the relative magnitude of 6

7 the odds of exposure among individuals who have the disease (cases) and the odds of exposure among individuals who do not have the disease (controls): OR = (a/c)/(b/d). From a typical 2 x 2 table: Cases Noncases Exposed a b Nonexposed c d Odds of exposure among cases: a/c Odds of exposure among controls: b/d Odds ratio = (a/c)/(b/d) Hence, OR = 1 means that the odds of exposure among cases are the same as the odds among the controls. Thus there is no evident association between exposure and disease. An OR > 1 denotes larger odds of exposure among the cases than among the controls. Thus individuals who have the disease (cases) have a higher odds of having been exposed in the past than individuals without the disease (controls). This situation illustrates a risk factor. With the same reasoning, an OR < 1 denotes smaller odds of exposure among individuals with the disease as compared to controls, suggesting that controls have a higher odds of having been exposed. Therefore, this situation illustrates a protective factor. Onset (disease onset) Prevalence Relative risk Beginning of the disease or condition under study. The prevalence is a proportion in which the numerator includes existing cases of a disease or condition, and the denominator includes the population from which the cases of disease came. The result is often multiplied by a multiple of 10 to obtain a convenient number. The relative risk (RR) is a measure of association between a disease or condition and a factor under study. It is calculated by dividing the incidence rate of those exposed to the factor by the incidence rate of those not exposed to the factor. RR = Incidence in the exposed Incidence in the nonexposed 7

8 RR cannot generally be calculated in a case-control study because the entire population has not been studied, so incidences are unkown. From a 2 x 2 table of a cohort study the relative risk could also be calculated as follows: Cases Noncases Exposed a b Nonexposed c d Incidence in the exposed: a/(a + b) RR Incidence in the nonexposed: c/(c + d) Incidence in the exposed a/(a+ b) = = Incidence in the nonexposed c/(c+ d) The RR is a measure of the relative magnitude of the incidence in the exposed and the incidence in the nonexposed. RR = 1 means that the incidence in the exposed is the same as the incidence in the nonexposed, and so there is no evident association between exposure and disease. RR > 1 denotes a larger incidence in the exposed than the nonexposed. Thus exposure to the factor seems to increase the probability of developing the disease. With the same reasoning, RR < 1 denotes a smaller incidence in the exposed as compared to the nonexposed. Thus exposure to the factor seems to decrease the probability of developing the disease. 8

9 Notes for Teachers Definition of a Case-Control Study A case control study is an investigation that compares a group of people with a disease to a group of people without the disease. Like many research designs, it is used by epidemiologists to identify and assess factors that are associated with diseases or health conditions, with the ultimate goal of preventing such diseases. A case control study begins with a group of cases of a specific disease or condition. A group of people without that disease or condition is selected as control, or comparison, subjects. The investigator then seeks to compare cases and controls with respect to previous exposures to factors of interest. Information about prior exposure may be obtained by a variety of methods, including self-administered questionnaires, interviews and medical examinations. Because in case control studies, information about exposure is generally collected after the disease has already occurred, these studies are sometimes called retrospective studies. Case control studies start with the outcome and look backward for the exposure, unlike cohort studies, which start from the exposure and look forward for the outcome. For example, a case control study of asthma in high school students may identify a group of students who suffer from asthma and compare them to a control group of students without asthma in regard to factors such as presence of carpets in the house, presence of household pets and family history of asthma. If the odds of exposure to one of these factors are different in the cases and controls, then that factor is associated with asthma and may influence the occurrence of the disease. Identification of Cases To conduct a case control study, we must start by identifying a group of people who have the disease in question, typically called cases. One may identify cases through hospital registries or clinic records listing all patients having a certain disease. We can also locate cases through local health department disease registries. Cases can also be found in a predefined group that includes medical records, such as high schools and some industrial plants, or in a prepaid health insurance group. Identification of Controls Once individuals with the disease under study (cases) are identified, individuals without the disease (controls) should be identified. Selection of controls for case control studies is one of the 9

10 most difficult design issues in epidemiology because the apparent difference in prior exposure of cases and controls may be thought to be the result of the factors that cause the disease but may actually be the result of the process used to select the controls (an error called selection bias). Controls should be representative of the source population from which cases were derived. Although challenging and often expensive, the surest approach is to draw a random sample of controls from the source population from which the cases came. Other sources of control groups include special groups such as friends, neighbors or relatives of the cases. Hospital- or clinic-based controls are frequently used but often are not representative of the source population. This happens when the reasons for attending the hospital or clinic may be different for cases and controls. For example, in a case control study investigating the social and economic factors associated with depression, a certain clinic may be known to have the best physicians specializing in depression in a region. However, physicians with other specialties at the clinic may be no better than their peers in that region. If this is so, and cases and controls are both selected from that clinic, cases of depression may be representative of the whole region, whereas the controls represent only the local neighborhood, which may have different social and economic characteristics. Analysis of the Data Once the person planning a case control study has identified the outcome of interest (disease or health condition) and the factors to be studied, a method for collecting information (e.g., a self-administered questionnaire, an interview form or a medical examination form) is developed. Data should include information about the outcome of interest (e.g., presence or absence of asthma) and the factors under study (e.g., presence or absence of carpeting, presence or absence of household pets and presence or absence of a family history of asthma). Data analysis involves calculating the odds ratio as a measure of association between the disease and each of the factors of interest. The odds ratio can be used to determine if there is an association and to quantify the magnitude of such an association. Definition of an Odds Ratio The odds ratio (OR), sometimes called the relative odds, is a measure of association between exposures and the disease or outcome. It is frequently used in case control studies, in which incidence rates cannot be calculated. The odds ratio contrasts the odds of exposure among cases with the odds of exposure among controls. 10

11 Calculating the Odds Ratio In a case control study, investigators try to identify and assess factors that may influence the probability of developing the disease. A case control study of the relationship between smoking and lung cancer may be analyzed by using a 2 2 table as follows: Typical 2 2 Table for a Case Control Study Individuals With Individuals Without Lung Cancer Lung Cancer (Cases) (Controls) Smokers a b Nonsmokers c d Total a + c b + d Hence, a denotes persons with lung cancer (cases) who smoked, b denotes controls who smoked, c denotes persons with lung cancer (cases) who did not smoke, and d denotes controls who did not smoke. The first column lists the total number of persons with lung cancer (a + c) and the second column lists the total number of controls (b + d). The odds ratio contrasts the odds of exposure (smoking) among the cases (individuals with lung cancer) with the odds of exposure (smoking) among the controls (individuals without lung cancer). The odds of exposure among the cases is the ratio of cases with exposure to cases without exposure (a/c). The odds of exposure among the controls is the ratio of controls who are exposed to controls who are not exposed (b/d). Therefore the odds ratio is calculated as follows: OR = (a/c)/(b/d) = ad/bc Interpreting the Odds Ratio A hypothetical case control study in which 200 cases of lung cancer were compared to 200 controls regarding their smoking habits showed the following findings: 11

12 2 2 Table for a Case Control Study of Lung Cancer and Smoking Individuals With Individuals Without Lung Cancer Lung Cancer (Cases) (Controls) Smokers 127 (a) (b) 35 Nonsmokers 73 (c) (d) 165 Total Odds of exposure among cases: a/c = 127/73 = Odds of exposure among controls: b/d = 35/165 = Odds ratio = / = 8.2 The odds of exposure to smoking among cases of lung cancer are 8.2 times as large as the odds of smoking among controls. Therefore, the odds of smoking among individuals with lung cancer (cases) were 8.2 times as great as the odds among the controls, a magnitude that indicates an important association between lung cancer and smoking. Smoking could thus be a factor that increases the probability of having lung cancer. Factors that increase the probability of having the disease are called risk factors. However, we cannot calculate an incidence rate of a disease based on a case control study. This is because we do not know the size of the population from which the cases were drawn. The number of people exposed (a + b) and the number of people unexposed (c + d) are the arbitrary results of the number of cases and controls that were selected and not representative of any real population. Therefore, we cannot calculate the risk of a disease in the exposed and unexposed. As stated above, we can identify risk factors by calculating the odds of exposure among the cases and the controls and comparing them by calculating an odds ratio. It is important to realize that when this is done, we are comparing the odds in favor of exposure among the cases and controls [(a/c) / (b/d)]. Conceptually this is very different from comparing the odds in favor of disease among the exposed and unexposed [(a/b) / (c/d)], which is what we are really interested in being able to calculate. Fortunately, however, algebraically the odds in favor of exposure among the cases and controls [(a/c) / (b/d)] are equal to ad/bc. And one can see that the odds in favor of disease among the exposed and unexposed [(a/b) / (c/d)] are also equal to ad/bc. Therefore, the odds in favor of exposure among the cases and controls are equal to the odds in favor of disease among the exposed and unexposed, which is what we really wish to calculate. 12

13 2 2 Table for a Case Control Study of Lung Cancer and Smoking Individuals With Individuals Without Lung Cancer Lung Cancer (Cases) (Controls) Smokers 127 (a) (b) 35 Nonsmokers 73 (c) (d) 165 Total Odds of exposure among cases: a/c = 127/73 = Odds of exposure among controls: b/d = 35/165 = Odds ratio = / = 8.2 The odds of exposure to smoking among cases of lung cancer are 8.2 times as large as the odds of smoking among controls. Therefore, the odds of smoking among individuals with lung cancer (cases) were 8.2 times greater than the odds among the controls, a magnitude that indicates an important association between lung cancer and smoking. Smoking could thus be a factor that increases the probability of having lung cancer. Factors that increase the probability of having the disease are called risk factors. However, we cannot calculate an incidence rate of a disease based on a case control study. This is because we do not know the size of the population from which the cases were drawn. The number of people exposed (a + b) and the number of people unexposed (c + d) are the arbitrary results of the number of cases and controls that were selected and not representative of any real population. Therefore, we cannot calculate the risk of a disease in the exposed and unexposed. As stated above, we can identify risk factors by calculating the odds of exposure among the cases and the controls and comparing them by calculating an odds ratio. It is important to realize that when this is done, we are comparing the odds in favor of exposure among the cases and controls [(a/c) / (b/d)]. Conceptually this is very different from comparing the odds in favor of disease among the exposed and unexposed [(a/b) / (c/d)], which is what we are really interested in being able to calculate. Fortunately, however, algebraically the odds in favor of exposure among the cases and controls [(a/c) / (b/d)] are equal to ad/bc. And one can see that the odds in favor of disease among the exposed and unexposed [(a/b) / (c/d)] are also equal to ad/bc. Therefore, the odds in favor of exposure among the cases and controls are equal to the odds in favor of disease among the exposed and unexposed, which is what we really wish to calculate. 12

14 Sometimes the factor under study can decrease the probability of having the disease. Such factors are called protective factors. Their association can also be measured with an odds ratio. For example, consider a hypothetical case control study in which 250 obese individuals (cases) were compared to 250 nonobese individuals (controls) regarding the consumption of vegetables in their diets. The results showed the following: 2 2 Table for a Case Control Study of Obesity and Regularly Eating Vegetables Obese Individuals (Cases) Nonobese Individuals (Controls) Eat Vegetables 121 (a) (b) 171 Do Not Eat 129 (c) (d) 79 Vegetables Total Odds of exposure among cases: a/c = 121/129 = Odds of exposure among controls: b/d = 171/79 = Odds ratio = / = 0.43 The odds of exposure to eating vegetables among cases were 0.43 times as large as the odds of that exposure among controls. Therefore, the odds of exposure to eating vegetables among cases were 57% smaller than the odds of that exposure among controls. The results of this study show that eating vegetables was less likely to be found among cases than among controls. Eating vegetables thus could be a protective factor decreasing the probability of being obese. Hence, the odds ratio can also measure how much smaller the odds of exposure are among cases as compared to controls. Sometimes during a study no association is found between the study factor and the disease. This situation can also be measured with the odds ratio. Consider the results of a hypothetical case control study in which 220 individuals suffering from depression (cases) were compared to 220 individuals not suffering from depression (controls) regarding the consumption of vegetables in their diets: 13

15 2 2 Table for a Case Control Study of Depression and Eating Vegetables Individuals With Depression (Cases) Individuals Without Depression (Controls) Eat Vegetables 90 (a) (b) 90 Do Not Eat 130 (c) (d) 130 Vegetables Total Odds of exposure among cases: a/c = 90/130 = Odds of exposure among controls: b/d = 90/130 = Odds ratio = / = 1.0 The odds of exposure to eating vegetables among the individuals with depression (cases) are the same as the odds of exposure among controls. The odds ratio thus was found to be equal to 1.0, denoting a lack of association between the outcome (depression) and eating vegetables. The results of this study do not show an association between eating vegetables and having depression. Therefore an OR > 1 suggests a possible risk factor, an OR < 1 suggests a possible protective factor and an OR = 1 suggests no association. Strengths of the Case Control Study Case control studies have several advantages. They can be used to study infrequent (rare) diseases and are relatively inexpensive because no follow-up is necessary. Exposure is then ascertained retrospectively. The investigator does not have to wait for the accumulation of enough individuals who are developing the disease, as in cohort studies. For the same reason, case control studies are particularly useful for studying diseases with long incubation or latency periods. Because they are small and retrospective, they are often cheaper to do than cohort studies and randomized controlled trials. Limitations of the Case Control Study Unlike cohort studies, case control studies cannot be used to compute incidence rates. Without knowing incidence, it is not possible to compute the relative risk. One can, however, use 14

16 case control studies to compute the odds ratio, a measure of association that under certain conditions approximates the relative risk. (These conditions will be explored later in the module.) The chronologic order of the exposure and disease, which is easy to elucidate in cohort studies, may be uncertain from the results of a case control study because it may not be possible to know if the exposure occurred before the disease. For example, if a case control study of asthma in high school students demonstrates an association with cat ownership, it may be difficult to know what happened first: the presence of cats or the first asthma attack. One can often overcome this problem by including only newly diagnosed cases. In practice, case control studies are often affected by selection bias. Selection bias may be present if the control group does not come from the same population as the cases. For example, if cases of asthma are drawn from a population of high school students, and controls without asthma are drawn from a population of older individuals who do not attend high school, one risks introducing a serious bias. This represents a problem because the factors related to asthma are different in young and older individuals. Consequently, many factors that might be found to be associated with asthma, based on a study of such mismatched cases and controls, might result merely from the fact that these two populations are of different age. Information bias is another common problem of case control studies. A type of information bias frequently found in case control studies is recall bias. Recall bias may arise when individuals with a disease (cases) remember past exposures more completely (or less completely) than controls. This often happens because sick individuals try to figure out what caused their disease. Therefore, when they are interviewed they often remember more events from their past. In contrast, individuals without disease (controls) have no special reason to remember past exposures. Although case control studies are good for studying rare diseases, they are not generally good for studying rare exposures. For example, if we want to study the risk of asthma from working in a nuclear submarine shipyard, we would probably not do a case control study because a very small proportion of people with asthma would have been exposed to such a factor. Case control studies cannot be used to study multiple diseases or conditions because it is necessary to guarantee that the control group is comparable for each disease or condition selected. Therefore, if one wants to study more than one disease, new groups of cases and controls are needed. 15

17 Teacher s Narrative Class 1 As Mr. Wilson monitored the 3 5 p.m., after-school, make-up-homework hall, he observed that many of the 20 students who were there seemed exhausted. Only a few of the students were working on making up their homework, in spite of the fact that when their homework was completed, they could leave. Mr. Wilson also noticed what seemed to him like an abundance of cell phones on students desks and belts. When Mr. Wilson asked how many students had cell phones, 15 students raised their hands. Mr. Wilson wondered if the reason why so many of these students had not done their homework was because they spent too much time talking on their cell phones. Mr. Wilson, who is a history teacher, was behaving like a scientist, in particular like an epidemiologist. He was observing a group of people, counting the exposures and outcomes that people experience as they go about their daily lives, and hypothesizing: Talking too much on a cell phone causes students not to do their homework. Epidemiologists have a tool for displaying the results of their studies, called a contingency table. The most basic contingency table is called a 2 2 table because it has two rows and two columns. The two rows are for identifying the people who were and were not exposed to the hypothesized cause of the outcome, and the two columns are for identifying the people who have and do not have the outcome. (Transparency 1) Outcome No Outcome Exposed a b Not Exposed c d Total Notice that four cells are formed where the two rows and two columns cross. Cell a is for people who were exposed (used cell phones) and had the outcome (did not do homework), cell b is for people who were exposed (used cell phones) and did not have the outcome (did homework), cell c is for people who were not exposed (did not use cell phones) and had the outcome (did not do homework), and cell d is for people who were not exposed (did not use cell phones) and did not have the outcome (did homework). Give students the case control study Worksheet A and ask them to begin to answer Question 1 by labeling the 2 x 2 table to test Mr. Wilson s hypothesis: Talking too much on a cell phone causes 16

19 Ask students to answer Question 2 by calculating what the odds of having had a cell phone were among Mr. Wilson s sample of students who had not done their homework. (15 to 5 or 3 to 1) Ask students if the odds of 3 to 1 support Mr. Wilson s hypothesis: Talking too much on a cell phone causes students not to do their homework. Probe until students uncover that it depends on what the odds of cell phone use were among a group of students who had done their homework. If necessary, refer students back to Transparency 3. Knowing that the students who were practicing for the spring concert would not have been able to practice if they had not done their homework, Mr. Wilson walked down the hall to the room in which the school orchestra was practicing and asked the 40 students who were practicing whether or not they had cell phones. Thirty students raised their hands. Ask students to continue to answer Question 1 by placing these counts in Mr. Wilson s 2 x 2 table. (Transparency 4) Did Not Do Did Homework Homework Had Cell Phone 15 a b 30 Did Not Have 5 c d 10 Cell Phone Total Ask students to answer Question 3 by calculating what the odds of having had a cell phone were among the students who had done their homework. (30 to 10 or 3 to 1) Ask students whether this hypothesis would have been supported if the odds of cell phone use among a group of students who had done their homework were 3 to 1. (No, the odds are the same.) Ask students if the counts presented in the 2 x 2 table below support Mr. Wilson s hypothesis. (Yes)(Transparency 5) Did Not Do Did Homework Homework Had Cell Phone 15 a b 10 Did Not Have 5 c d 30 Cell Phone Total

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