The Cox proportional hazards model, introduced in 1972,1 has become the default approach for survival analysis in randomized trials. The Cox model estimates the ratio of the hazard of the event or outcome of interest (eg, death) between 2 treatment groups. Informally, the hazard at any given time is the probability of experiencing the event of interest in the next interval among individuals who had not yet experienced the event by the start of the interval. Because the Cox model requires the hazards in both groups to be proportional, researchers are often asked to “test” whether hazards are proportional.
Stensrud MJ, Hernán MA. Why Test for Proportional Hazards? JAMA. Published online March 13, 2020. doi:10.1001/jama.2020.1267
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