Measures of disease frequency

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 disease monitoring systems, and is commonly the central aim of many 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.

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, 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 surveys or cross sectional studies, although in some cases, cohort studies may be appropriate (dependent on how animals were selected).

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.

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.

Incidence rate

The incidence rate is

Counts

Disease counts (i.e. the number of diseased animals at a point in time or over a period of time) provide a useful measure of the number of animals experiencing the disease in question. As such, they are commonly used when disseminating information to the general public, or when deciding upon the resources required to control or manage the disease. However, the figures given in disease counts are not shown in relation to the size of the population of interest (that is, there is no 'denominator'), and so alone do not provide sufficient information for epidemiological investigation or interpretation.