In a long-term comparative oncology trial, progression-free survival or overall survival time is often the study endpoint. The hazard ratio (HR) has been routinely utilized to quantify the between-group difference in survival analysis for the past five decades [1]. The validity of HR estimation procedure depends on a strong assumption of proportional hazards (PH), i.e. the ratio of the two hazard functions is constant over the entire study period. When the PH assumption is not met, the resulting estimate is not a simple average of HRs over time and is difficult to interpret clinically [2–6].
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Uno et al. (2018) studied this question.