How should response variables be defined in the protocol?

• A. They are determined by the investigator after risk assessment.
• B. They are selected solely based on regulatory expectations.
• C. They should be defined prospectively, including observation and quantification methods.
• D. They are derived from the primary endpoint only.

Answer: (C)

Defining response variables prospectively in the protocol means deciding in advance exactly what will be measured and how it will be measured. This includes specifying the observation methods, measurement tools or instruments, units of measurement, timing of assessments, and any scoring rules or thresholds. By prespecifying these details, the study ensures objective, reproducible endpoints and a clear plan for data collection and analysis, which also supports regulatory expectations.

Choosing endpoints or measurement methods after risk assessment introduces bias and the appearance of tailoring outcomes to favor results. Relying solely on regulatory expectations can overlook scientific relevance. And deriving variables only from the primary endpoint ignores other meaningful outcomes and the necessary measurement plan.