15 September 2020 - The goal of many medical research studies is to estimate the direction and magnitude of the effect of an intervention or treatment on a clinical outcome (in clinical trials) or the association between an exposure and an outcome (in observational studies). 

This effect or association can be presented in various forms, depending on the measured outcome. 

For example, if the outcome is a continuous measure (eg, blood pressure), the effect or association could be represented as a mean difference between the groups. If the outcome is a time-to-event outcome (eg, time to death), the effect or association is often expressed as a hazard ratio.

Read JAMA Guide to Statistics and Methods