Which metric is commonly used to compare the probability of an event between treatment and control groups in a binary outcome trial?

• A. Mean difference
• B. Hazard ratio
• C. Risk ratio
• D. Spearman rho

Answer: (C)

In binary outcomes, you’re looking at the probability (risk) that an event happens in each group. The metric that directly compares those probabilities is the risk ratio, which is the risk in the treatment group divided by the risk in the control group. It tells you how many times more or less likely the event is with treatment. For example, if 20% experience the event with treatment and 40% with control, the risk ratio is 0.5, meaning the treatment halves the risk.

Mean difference is suited for continuous outcomes, not probabilities. Hazard ratio applies to time-to-event data and accounts for censoring. Spearman rho measures rank correlation, not group-wise event probabilities.