Community Health M3 Practice Test

Risk ratio is about comparing the probability of an outcome between two groups over time, so you need incidence data from a study that follows people prospectively. A randomized controlled trial provides exactly that: random assignment to an intervention or control and then tracking who develops the outcome. With this setup you can directly compute the risk in each arm and form the ratio, giving a clear measure of how the intervention changes risk while preserving internal validity from randomization.

In contrast, case-control studies start with people who do and do not have the outcome, so you don’t observe incidence to calculate risk; you typically estimate an odds ratio instead. Cross-sectional studies give a single point in time, offering prevalence rather than incidence, which makes a direct risk ratio less appropriate. Case series have no comparison group at all, so a risk ratio cannot be computed.