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===

===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]]).

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). Although the hazard function may be described graphically, it 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]]