The Primary Outcome in a study is defined as which of the following?

• A. A secondary endpoint
• B. An exploratory outcome
• C. Main endpoint of interest
• D. A surrogate marker

Answer: (C)

The primary outcome is the main endpoint of interest—the result the study is explicitly designed and powered to assess. This endpoint asks the central clinical question the trial aims to answer and often guides the sample size and primary analysis. Secondary endpoints are additional outcomes measured to learn about other effects but aren’t the main focus of the study. Exploratory outcomes are typically not pre-specified and are used to generate new hypotheses. A surrogate marker is a substitute measure that may predict the true clinical outcome; it can be used as a primary endpoint in some trials, but the standard definition centers on the main endpoint of interest.