THE ORIGIN OF TIME

All models for survival data are fundamentally concerned with the timing of events. In assigning a number to an event time, we implicitly choose both a scale and an origin. The scale is just the units in which time is measured: years, days, minutes, hours, or seconds. We have already seen that the numerical value of the hazard depends on the units of measurement for time. In practice, however, the choice of units makes little difference for the regression models discussed in later chapters. Because those models are linear in the logarithm of the hazard or event time, a change in the units of measurement affects only the intercept, leaving the coefficients unchanged.

The choice of origin (0 point) is more problematic, however, for three reasons: