Tutorials : proportional hazard assumption
An important assumption of the Cox model is the homogeneity of the effect of an explanatory variable over time. In other words, the hazards ratio (HR) is assumed to be constant over time. Several methods can be used to test this assumption.
Graphical approach
The log (-log (S (t))) curves - with S (t) the survival function and t, the time - can be plotted for each value of the qualitative variable. The difference between the curves must be constant over time. If the curves intersect, the effect of the variable of interest can be time-dependent.

