The Grambsch Therneau Test uses Schoenfeld residuals to check for proportionality. Schoenfeld residuals are the differences between observed and expected covariate values at each event time. If the proportional hazards assumption holds, these residuals should be independent of time. The test involves plotting these residuals against time and visually inspecting the plot for trends. A formal test statistic is also calculated, providing a p-value to determine if the assumption is violated.
