How do you compute risk ratio from a 2x2 table, and what does an RR of 1.5 indicate?

• A. RR = (a/c) / (b/d); RR > 1 indicates lower risk among exposed.
• B. RR = [a/(a+b)] / [c/(c+d)]; RR > 1 indicates higher risk among exposed; RR = 1.5 means 50% higher risk in exposed group.
• C. RR = (a+b)/(c+d); RR > 1 indicates higher risk among exposed.
• D. RR = (a+d)/(b+c); RR > 1 indicates higher risk among exposed.

Answer: (B)

Risk ratio tells you how much more (or less) likely an outcome is in the exposed group compared with the unexposed group. In a 2x2 table, the exposed group has a total of a + b people, with a experiencing the outcome and b not. The unexposed group has c + d people, with c experiencing the outcome and d not. The risk in the exposed is a/(a + b), and the risk in the unexposed is c/(c + d). The risk ratio is the division of these two risks: RR = [a/(a + b)] / [c/(c + d)]. An RR of 1.5 means the exposed group has 1.5 times the risk of the outcome compared with the unexposed group, i.e., about 50% higher risk.

The other formulas don’t compute the risk ratio from a 2x2 table. For example, (a/c) ÷ (b/d) equals (a d)/(b c), which is the odds ratio, not the risk ratio. The remaining options don’t reflect incidence in each group, so they don’t yield a proper risk ratio or a valid interpretation.