  The hazard ratio is one of the most commonly reported measures of treatment
effect in randomised trials, yet the source of much misinterpretation. This
point was made clear by (Hernan, 2010) in commentary, which emphasised that the
hazard ratio contrasts populations of treated and untreated individuals who
survived a given period of time, populations that will typically fail to be
comparable - even in a randomised trial - as a result of different pressures or
intensities acting on both populations. The commentary has been very
influential, but also a source of surprise and confusion. In this note, we aim
to provide more insight into the subtle interpretation of hazard ratios and
differences, by investigating in particular what can be learned about treatment
effect from the hazard ratio becoming 1 after a certain period of time.
Throughout, we will focus on the analysis of randomised experiments, but our
results have immediate implications for the interpretation of hazard ratios in
observational studies.
