Changes

===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). 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.

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 disease does not occur more than once for any individual, 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. Animals lost from the study over the study period 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===

The incidence rate is  

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.

 

==Time to event==

==Time to event==