Calling time on survival times?
Survival time has been used as a measure of treatment effect in many companion animal studies since the mid-1970s. For an individual animal, survival time is defined as the time from a starting point, typically disease diagnosis, until the occurrence of a critical event, which is often, but not always, death. For a group of animals, survival is typically summarised by the median survival time – the point at which 50% of the animals remain alive. The obvious reason for choosing death as an end-point is because it is easily defined and the date at which it occurs can usually be unequivocally determined, even in a retrospective study. It is also highly clinically relevant: owners are interested in knowing how long a pet will live. Alternative objective outcomes can be difficult to define or measure; e.g., it may be impossible to determine when an adrenal mass reaches a critical size if owners decline repeated imaging. But is the length of survival until death really as objective or useful as we would like to believe?
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