Changes

==Survival time analysis==

==Survival time analysis==

Survival analysis (also known as the '''time-to-event''' analysis) includes similar concepts to both the incidence risk and the incidence rate, but adopts a different approach to investigation and interpretation. Survival time analysis can be recognised as comprising two different measurements: the '''survivor function''' (and '''survival time''') and the '''hazard function'''. As for incidence measurement, these measures are based upon following an initially disease-free population of animals over time, and so require studies such as [[Study design#Cohort studies|cohort studies]].

Survival analysis (also known as the '''time-to-event''' analysis) includes similar concepts to both the incidence risk and the incidence rate, but adopts a different approach to investigation and interpretation. Survival time analysis can be recognised as comprising two different measurements: the '''survivor function''' (and '''survival time''') and the '''hazard function'''. Of these, the survivor function is most commonly used as a method of data description, with the hazard function being used for comparison of disease experience between groups in the case of analytic studies. As for incidence measurement, these measures are based upon following an initially disease-free population of animals over time, and so require studies such as [[Study design#Cohort studies|cohort studies]].

===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===

 

[[Category:Veterinary Epidemiology - Statistical Methods|B]]

[[Category:Veterinary Epidemiology - Statistical Methods|A]]