I'm a bit confused as it relates to the hazard ratio.
It seems to me that the hazard ratio is a time varying quantity however it seems that studies report a single number.
Read this https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5045282/ and it seems to imply that a hazard ratio is assumed constant? I find that very hard to believe with real world data - what is the approximation used?
Hazard ratios change over time and are reflected in the slope of the K-M plot. Reported hazard ratios assume that the differences between groups are a constant distance apart (i.e., the K-M survival curves) and are proportional
And from wiki (https://en.wikipedia.org/wiki/Hazard_ratio) it is stated that the hazard ratio is the ratio of the hazard rates which makes sense but again points to a time varying quantity.