Changes

==Prevalence==

==Prevalence==

The prevalence (also known as the ''prevalence risk''') of a disease is the proportion of animals in the population of interest which are 'diseased' at any specific point in time. This may be a single point (in which case, the measure is the '''point prevalence'''), or may be over a period (in which case the '''period prevalence''' is given). However, it is important to realise that this measure will therefore include both old and new cases of disease, which can be a problem when attempting to investigate risk factors for a disease of long duration (as many of the diseased animals are likely to have had the disease for some time). The prevalence can be viewed at an individual animal level as the 'probability that an animal selected at random from the population will be diseased'. The prevalence of disease is commonly obtained from [[Study design#Surveys|surveys]] or [[Study design#Cross sectional studies|cross sectional studies]], although in some cases, [[Study design#Cohort studies|cohort studies]] may be appropriate (dependent on how animals were selected).

The prevalence (also known as the ''prevalence risk''') of a disease is the proportion of animals in the population of interest which are 'diseased' at any specific point in time, and is calculated by dividing the number of cases of disease by the total size of the population. This may be a single point (in which case, the measure is the '''point prevalence'''), or may be over a period (in which case the '''period prevalence''' is given). However, it is important to realise that this measure will therefore include both old and new cases of disease, which can be a problem when attempting to investigate risk factors for a disease of long duration (as many of the diseased animals are likely to have had the disease for some time). The prevalence can be viewed at an individual animal level as the 'probability that an animal selected at random from the population will be diseased'. The prevalence of disease is commonly obtained from [[Study design#Surveys|surveys]] or [[Study design#Cross sectional studies|cross sectional studies]], although in some cases, [[Study design#Cohort studies|cohort studies]] may be appropriate (dependent on how animals were selected).

==Incidence==

==Incidence==

===Incidence risk===

===Incidence risk===

The incidence risk (also known as the ''cumulative incidence''') is the proportion of nondiseased animals which develop disease over a particular period of time, and so only relates to the period of time in question (meaning that this period must always be stated along with the risk estimate).  

The incidence risk (also known as the '''cumulative incidence''') is the proportion of nondiseased animals which develop disease over a particular period of time, and so only relates to the period of time in question (meaning that this period must always be stated along with the risk estimate). It is calculated as the number of animals which develop disease over the period in question divided by the number of nondiseased animals at the start of the study. This measure assumes that no animals are added to the population over the period of investigation, and that if animals are removed from the population over this period without experiencing disease, that these were lost half way through the study (these animals are accounted for in the calculation of incidence by subtracting half an animal per animal lost from the total number of nondiseased animals at the start of the study). At the individual animal level, the incidence risk can be interpreted as the probability that a nondiseased animal will develop the disease over the period of time in question.

===Incidence rate===

===Incidence rate===

==Time to event==

==Time to event==