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===Survivor function===
===Survivor function===
The survival function (also known as the '''cumulative survival probability''') describes the proportion of initially disease-free individuals who have not experienced the outcome of interest over time, and so is similar conceptually to the incidence risk. The survival time may be visualised using a Kaplan-Meier curve, or may be summarised using the ''survival time''. Survival time is measured in units of time and describes the time until animals experience disease. Commonly, the '''median survival time''' is presented, which is the time at which 50% of the population have experienced the event (this is the most appropriate measure of the 'average' survival time, as the mean would not be able to account for individuals who have not experienced the outcome by the end of the study). However, if 50% of the individuals have not experienced disease by the end of the study, the median survival time cannot be calculated. Another option is to describe the proportion of individuals who have not experienced disease at a set point in time, which is the same concept as the [[#Incidence risk|incidence risk]]
The survival function (also known as the '''cumulative survival probability''') describes the proportion of initially disease-free individuals who have not experienced the outcome of interest over time, and so is similar conceptually to the incidence risk. The survival time may be visualised graphically (using a Kaplan-Meier curve, for example) or may be summarised using the ''survival time''. Survival time is measured in units of time and describes the time until animals experience disease. Commonly, the '''median survival time''' is presented, which is the time at which 50% of the population have experienced the event (this is the most appropriate measure of the 'average' survival time, as the mean would not be able to account for individuals who have not experienced the outcome by the end of the study). However, if 50% of the individuals have not experienced disease by the end of the study, the median survival time cannot be calculated. Another option is to describe the proportion of individuals who have experienced/not experienced disease at a set point in time, which is the same concept as the [[#Incidence risk|incidence risk]].
===Hazard function===
===Hazard function===
The hazard function describes the pattern of the 'incidence rate' over time (rather than summing an 'average' estimate over the duration of the study, as with the incidence rate itself). It may be described graphically, but is most commonly used in analytic studies, when hazard functions are compared in the form of a hazard ratio (a [[Measures of strength of association|measure of strength of association]]).
[[Category:Veterinary Epidemiology - Statistical Methods|A]]
[[Category:Veterinary Epidemiology - Statistical Methods|A]]