What does Relative Risk measure when calculated from a 2x2 table?

• A. A 2x2 table
• B. A time series chart
• C. A matched-pairs design
• D. A Kaplan-Meier curve

Answer: (A)

Relative risk measures how likely the outcome is in the exposed group compared with the unexposed group. In a 2x2 table, you have the counts of people with and without the outcome for both exposure groups. The risk in the exposed group is the number with the outcome among the exposed divided by the total exposed, and the risk in the unexposed group is the number with the outcome among the unexposed divided by the total unexposed. Relative risk is that ratio of the two risks, so the 2x2 table is exactly what you use to compute it. This differs from a time series chart (which shows data over time), a matched-pairs design (which involves paired data), or a Kaplan-Meier curve (which estimates survival over time).