Proportional-hazards time interaction
= Proportional-hazards time interaction
{title2=$\beta_k(t)=\beta_k+\gamma_k g(t)$}
A time interaction extends a constant-coefficient <Cox proportional-hazards model> by a covariate term $z_kg(t)$ and tests whether its coefficient is zero. The time function is evaluated at the current event/risk-set time, not each person's eventual outcome time. <Scaled Schoenfeld residuals> provide related graphical and score-test diagnostics for coefficient trends.