Changes

A very common aim of epidemiological investigation is to estimate the levels of disease in a population. This is of particular importance in the case of [[Surveillance and monitoring of disease|surveillance and disease monitoring systems]], and is commonly the central aim of many [[Descriptive epidemiological studies|descriptive studies]]. There are two main measures of disease frequency used by epidemiologists - the '''prevalence''' and the '''incidence''' of disease, which each measure different aspects of disease. The '''time to event''', which is closely related to the incidence rate, is another measure commonly used. Counts of disease are not commonly used in epidemiological studies, although they can be useful when deciding upon resource requirements when implementing disease control strategies.

A very common aim of epidemiological investigation is to estimate the frequency of disease in a population. This is of particular importance in the case of [[Surveillance and monitoring of disease|surveillance and disease monitoring systems]], and is commonly the central aim of many [[Descriptive epidemiological studies|descriptive studies]]. There are two main measures of disease frequency used by epidemiologists - the '''prevalence''' and the '''incidence''' of disease, which each measure different aspects of disease. The '''survival time''', which is closely associated with the incidence, is another measure commonly used. Counts of disease are not commonly used in epidemiological studies, although they can be useful when deciding upon resource requirements when implementing disease control strategies.

==Prevalence==

==Prevalence==

==Incidence==

==Incidence==

The incidence of disease describes the frequency of new cases of disease amongst previously nondiseased animals, and there are two main methods of measuring it - known as the '''incidence risk''' and the '''incidence rate'''.

The incidence of disease describes the frequency of new cases of disease amongst previously nondiseased animals, and there are two main methods of measuring it - known as the '''incidence risk''' and the '''incidence rate'''. The incidence can only be measured from studies which follow animals up over time, such as [[Study design#Cohort studies|cohort studies]] and [[Study design#Experimental studies|randomised controlled trials]].

===Incidence risk===

===Incidence risk===

===Incidence rate===

===Incidence rate===

The incidence rate is a measure of the frequency of disease in relation to both the number of nondiseased animals and the total amount of time for which these animals were followed, meaning that the time period does not need to be provided in order for it to be interpretable. It can also be viewed as the 'instantaneous rate of disease' at any point over the period of investigation. It is calculated by dividing the number of cases of disease over the period of study with the total number of '''animal-time units''' which were contributed over this period. These animal-time units are estimated by adding together the total amount of time which each animal was present in the study for (i.e. until animals experienced disease, left the study for other reasons, or the study concluded) for each animal in the study. Examples of animal-time units may be cow-days, or dog-weeks, or horse-years, and so on. As each individual animal contributes its own number of animal-time units, animals which enter the study population during the study period can be included, as can animals which experience more than one episode of disease (in which case, it must be ensured that animals which have experienced disease and are susceptible again contribute animal-time units to the denominator). Unlike the prevalence and the incidence risk, the incidence rate has no interpretation at the individal animal level, only at the population level.  

The incidence rate is a measure of the frequency of disease in relation to both the number of nondiseased animals and the total amount of time for which these animals were followed, meaning that the time period does not need to be provided in order for it to be interpretable. It can also be viewed as the 'instantaneous rate of disease' at any point over the period of investigation (and therefore assumes that this rate is constant over the period). It is calculated by dividing the number of cases of disease over the period of study with the total number of '''animal-time units''' which were contributed over this period. These animal-time units are estimated by adding together the total amount of time which each animal was present in the study for (i.e. until animals experienced disease, left the study for other reasons, or the study concluded) for each animal in the study. Examples of animal-time units may be cow-days, or dog-weeks, or horse-years, and so on. As each individual animal contributes its own number of animal-time units, animals which enter the study population during the study period can be included, as can animals which experience more than one episode of disease (in which case, it must be ensured that animals which have experienced disease and are susceptible again contribute animal-time units to the denominator). Unlike the prevalence and the incidence risk, the incidence rate has no interpretation at the individal animal level, only at the population level.  

==Time to event==

==Survival time==

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. Three approaches to survival time analysis can be recognised: measurement of the '''survival time''', measurement of the '''survivor function''' and measurement of the '''hazard function'''. As for incidence measurements, these measures are based upon following an initially disease-free population of animals.

 

 

==Hazard function==

==Counts==

==Counts==