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Measures of Disease Occurrence
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## Learning Objectives:

After reading this chapter, you will be better able to:

- appreciate the definition of risk, rate, prevalence and odds;
- appreciate the difference between measures of disease occurrence and measures of effect;
- familiarize with the different terms and synonyms which are used to describe risk, rates, prevalence;
- understand that the epidemiological jargon is not always correct (an attack rate is actually a risk);
- identify what is the best measure to calculate in different study designs.

## Disease occurrence

The measures to be used depend on the study design, but also on what we want to measure. Measures of disease occurrence are used when we are interested only in quantifying an event (the outcome), and our analysis does not extend further to take into account exposures. When we want to relate the effect of a certain exposure to an outcome, we will then need to use what we call measures of effects.

A risk represents a proportion of the number of people developing the disease divided by the number of people in the population. It can be presented as a proportion (ranging from 0 to 1) or as a a percentage (ranging from 0% to 100%), and expresses the probability of an outcome (health event, disease etc) in a certain group. Although time units are not expressed, the concept of risk implies that we are observing a population for a specific time period. Risks may also be expressed per 10,000 or per 100,000 population; this is sometimes called '**cumulative incidence**', or simply '**incidence**'.

Rates are used when the occurrence of an event relates to units of time (for example the number of deaths per 100 persons-years). When measuring the occurrence of a new event in relation to units of time, we refer to incidence rates.

In cohort studies performed during an outbreak investigation, attack rates are often calculated to have a measure of the proportion of people who experience the outcome of the study. Indeed, attack rates (as well as case fatality rates or ratio) are risks and are an example on how epidemiological jargon might be misleading. Though they are called rates, they are proportions because they do not relate to units of time.

Sometimes it might be convenient to approximate rates into risks, as the interpretation of risks is easier.

Prevalence is a proportion of how many events (for example, people with disease) are present at a specific point in time in a population. It is expressed as a percentage.

The odds of an event ("odds", always plural) are the probability that this event will occur divided by the probability that the event will not occur. Therefore, a value of the odds of the event occurring can range from 0 to infinity. Odds are a measure rarely used, though the ratio of two odds (odds ratio) are one of the most commonly used measures of effect in epidemiology.

Whenever using any of these measures it is important to consider the context in which they are used and whether or not they properly express what we want to measure.

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