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Cumulative incidence measures the proportion of the population at risk that develops the disease over a specified time period . Given it is a proportion it can be expressed as a percentage .
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Cumulative incidence |
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number of new cases of disease over a specified period
number of animals in the source population at risk
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The cumulative incidence does not account for situations where there is a large change in numbers of the population at risk ( with additions and withdrawals ). To handle such fluctuations , you can average the population over the time period ( e . g . month ) or reduce the specified time period as demonstrated in the “ Happy Abs ” example ( see page 12 ).
We need to understand incidence because we often use a measure known as the attack rate when looking at disease outbreaks . The attack rate is a subtype of cumulative incidence and is the proportion of a specific population affected during an outbreak . Comparing attack rates in different groups can help us look at potential factors that may be responsible for causing the outbreak . This will be explored further in Step 7 ( see “ Step 7 Analyse the data ” on page 46 ).
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Attack rate percentage |
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number of new cases since onset of outbreak X 100
total number of animals at risk at onset of outbreak
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Photo . Southern rock lobster ( Photo courtesy of Moon Cheese Studio )
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