Design and Analysis of Clinical Study 6. Case-control Study Dr. Tuan V. Nguyen Garvan Institute of Medical Research Sydney, Australia

What is Case-Control ? Traditional view: compare - people who get the disease - people who do not get the disease “Controls” a misnomer, derived from faulty analogy to controls in experiment Modern conceptualization: controls are a “window” into the “study base”

Case – Control Study Grouping studied: "cases" vs. "Control" group(s). Measurements analyzed: past "exposures.“ Case-selection usually clinic- or hospital-based. Controls may also be clinic- or hospital-based, or population-sampled. Controls may be unmatched, group-matched, or individually matched.

Case-Control Study Cases Controls Population at risk Yes No Yes No

Case-Control Study Cases Controls Population at risk Aspirin use No Aspirin use Aspirin use No Aspirin use

Steps in Designing Case-control Studies Selection of cases –Precise definition of ‘case’. –Inclusion / Exclusion criteria. –Are cases to be ‘incident’ or ‘prevalent’? –How are cases to be identified? How recruited?

Steps in Designing Case-control Studies Selection of Controls –Source ( hospital patients without disease; neighbourhood controls; random sample of population; sibs). –Inclusion / exclusion criteria. –Match to cases?

Steps in Designing Case-control Studies Collection of information –Identify risk factor of interest –Method of collection of information ( questionnaire; medical records; employment records) –Same procedure to be used for cases and controls –Interviewer should be unaware who is a case and who a control.

Two Methods of Selection Select new cases (i.e.incident) as they come up. Controls are selected from those in the same setting at the same time. Select existing cases (prevalent) from a defined population. From the same population a larger number of controls are identified. The Incident type of case-control study is stronger because diagnosis of cases and ascertainment of exposure is being done by the researcher.

Results of a Case-Control Study Risk factor Disease Yes (cases)No (controls) Aspirin Useac No Aspirin Usebd TotalN1N2 N1 and N2 are fixed numbers

Nested Case-Control Study Case-control studies within a cohort study In ARIC (Atherosclerosis Risk in Communities) study, a cohort of 16 thousand men, all men provided serum samples at the outset which were saved. The cohort is observed for CHD. After 5 years we have 246 cases of CHD. We randomly choose 500 participants to be controls. We only measure Chlamydia antibody in the stored sera from these subjects. We compare the cases (CHD) to the controls (no CHD) with regard to the presence of exposure (Chlamydia) which preceded the outcome

Matched Case-Control Studies cases Non-cases cases Non-cases All cases or random sample Cases Random sample of non-cases Cases All cases or random sample Controls Matched controls

Effects of Beta-blocker on Hip Fracture Select a hip fracture case Note the patient’s age, sex, weight, bone mineral density (BMD) Select a sample of controls Randomly selected k controls who have the same age, weight, and BMD as the case

Potential Biases A knowledge of the patient's disease status may influence: –Both the intensity and outcome of a search for exposure to the putative cause A late look at those exposed (or affected) early will miss: –Fatal and other short episodes, plus mild or silent cases and cases in which evidence of exposure disappears with disease onset

COHORT VS. CASE-CONTROL STUDIES OF CHD VS. CHOLESTEREMIA AMONG MEN UPPER QUARTILE SERUM CHOLESTEROL COHORT STUDY CASE-CONTROL STUDY CHD BY EXAM 6 CHD BY EXAM 6 YESNOTOTALYESNOTOTAL YES NO TOTAL ODDS RATIO = 2.40ODDS RATIO = 1.16

Sample Size Calculation Power : probability of detecting a real effect (eg  = 0.20) Alpha level : probability of detecting a false effect (eg  = 0.05) P 0 : probability of exposure in controls P 1 : probability of exposure in case subjects R : odds ratio of exposures between cases and controls m : number of control subjects per case subject

Sample Size Calculation The estimated sample size is:

Website for Sample Size Calculation 20case%20control.html

Advantages and Disadvantages of Case-control Studies Relatively cheap compared to cohort studies Relatively quick Useful for study of rare diseases. No ethical problems Useful for diseases with long latent period. Estimate of disease incidence cannot be done At times difficult to measure exposure accurately Open to selection bias. Difficult to interpret. AdvantagesDisadvantages

Self-evaluation Questions Q2: Suppose that in a case-control study using incident cases of colon cancer you found that 80% of the cases were married. Does this demonstrate that being married increases the risk of developing cancer? Q2: In the same case-control study above, assume that 90% of the control group group are married. If there are 200 cases and 200 controls, estimate the risk of colon cancer for single men. Constuct a 2x2 table and determine and interpret the exposure odds ratio.