In a 2x2 table of disease status by exposure, how is relative risk calculated?

• A. RR = [a/(a+b)]
• B. RR = [a/(a+b)] / [c/(c+d)]
• C. RR = a/(a+d)
• D. RR = (a+c)/(b+d)

Answer: (B)

Relative risk compares the chance of developing the disease in those who are exposed to the chance in those who are not exposed. In a 2×2 table, the risk among the exposed is a divided by the total exposed (a+b), and the risk among the unexposed is c divided by the total unexposed (c+d). The relative risk is the ratio of these two risks: [a/(a+b)] / [c/(c+d)]. This is why that expression is the best answer. It directly compares the two group-specific risks. The other forms either give just one group's risk or mix counts in a way that doesn’t reflect the two-group comparison.