Which statement describes time-varying covariates?

• A. A covariate that changes across time (e.g., body condition).
• B. A covariate measured only at baseline.
• C. A covariate that remains constant.
• D. A covariate that cannot change.

Answer: (A)

Time-varying covariates are variables whose values can change during the observation period. For instance, body condition can change as health or nutritional status shifts, so it would be measured at multiple time points to reflect its current value at any moment. In analyses, this means the covariate can update over time and influence the outcome according to its value at each time point, such as in survival models where the hazard depends on the covariate’s current value. This differs from covariates measured only at baseline or those that remain constant, which are treated as fixed and do not reflect changes over follow-up. A covariate that cannot change would not describe time-varying covariates.