A time-varying covariate (also called time-dependent covariate) is a term used in statistics, particularly in survival analysis. It reflects the phenomenon that a covariate is not necessarily constant through the whole study Time-varying covariates are included to represent time-dependent within-individual variation to predict individual responses.
In the context of statistics and survival studies, a time-varying covariate represents a factor whose value changes over time. This concept recognizes that a covariate's value may shift throughout the research period. By including time-dependent covariates in models, researchers can better forecast individual reactions to various factors.