Brief overview of case control studies (part of a larger CE session)

1. Case-Control Studies Rachel Walden, MLIS Eskind Biomedical Library MLA 2007: Quality Filtering

2. “ Begin with the outcome and look for features of people who share that outcome, then compare characteristics with subjects who do not.” - Stephen H. Gehlbach, Interpreting the Medical Literature, 1993 Case-control studies start with a disease, and work backward to find associations between exposures and the disease. Compares patients with and without disease to determine how they may differ in what they were exposed to.

9. Case-Control Study Base Cases (50) Controls (50) Exposed (40) Not Exposed (10) Not Exposed (35) Odds Ratio: (number of exposed cases)/(number of unexposed cases) (number of exposed controls)/(number of unexposed controls) OR 40/10 X 35/15 = (4)(2.33)= 9.33 Odds of exposure for cases is 9.33 times that of controls Exposure is associated with 9x greater chance of disease Exposed (15)

10. Case-Control Study Base Cases (50) Controls (50) Exposed (40) Not Exposed (10) Not Exposed (35) Exposed (15) Odds Ratio: (40)(35) ÷ (15)(10) = 9.33 Can also be calculated by cross-multiplication of a 2x2 table Cases Controls Exposed Not Exposed 40 10 35 15

12. Our Example Study Authors may also look at other levels of exposure, like 1-19 pack years, or 0-5, 5-25, and ≥26 lifetime sexual partners Also look at factors that may be additive or synergistic, such as smoking and drinking, or smoking and exposure to asbestos Cases Controls Exposed Not Exposed 34 44 119 39 Smoking ≥20 pack-years Odds Ratio: (34)(119) ÷ (39)(44) = 2.4