Here, the survival time t of each patient is replaced by sequential time segments. As a consequence, instead of using the exact follow-up time we rounded it to the nearest in months. In the formula t is replaced by k the time segment in months and k₁, k₂ and k₃ indicate the time-segments in which the 3 knots are located. As we‘ve seen before there are only 3 parameters for each restricted cubic spline function and so there are 3 times p parameters for the p covariates. All these parameters are estimated by the full maximum likelihood method.