Which formula represents the odds ratio from a 2x2 table?

• A. ad/bc
• B. a/c divided by b/d
• C. (a+d)/(b+c)
• D. bc/ad

Answer: (A)

The odds ratio measures how much more likely exposure is among those with the disease compared to those without. In a 2x2 table, let a be the count who are exposed and diseased, b be exposed and not diseased, c be not exposed and diseased, and d be not exposed and not diseased. The odds of exposure among the diseased is a/b, and among the non-diseased is c/d. The odds ratio is the ratio of these two odds: (a/b) ÷ (c/d) = (ad)/(bc). This is the same as (a/c) ÷ (b/d), so the formula ad/bc correctly represents the odds ratio. For example, with a=30, b=10, c=15, d=45, OR = (30/10) ÷ (15/45) = 3 ÷ 1/3 = 9, which matches ad/bc = 30×45/(10×15) = 1350/150 = 9.