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| EPI2-6: Incidence Proportion Defined |
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| EPI2-6: Incidence Proportion Defined |
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OBJECTIVES |
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At the end of this session you should understand the meaning of incidence proportion as a measure of occurrence, when it can be measured and how to calculate it. |
The next measure to be defined is called the incidence proportion, (IP). It is also known as cumulative incidence.
It is defined as:
Both numerator and denominator include only those individuals who at the beginning of the period of observation are free from the disease and therefore at risk to get it. The incidence proportion is, therefore, the proportion of individuals in the disease-free state at the beginning of the period that move to the disease state during the period. That is, the numerator is a subset of the denominator.
Simply stated, the incidence proportion is the proportion of healthy individuals who get the disease during a certain period.
Alternatively, it can be viewed as the average risk for the individuals in that population to get the disease during that period, because not all individuals get their disease at the same time. Being a proportion, like prevalence, the incidence proportion is dimensionless and can only take numeric values in the range from 0 to 1.
The length of the observation period directly affects the incidence proportion: the longer the period, the greater the incidence proportion. An extreme example would be the study of total mortality among newborn infants over, say, the ensuing 115 years. The incidence proportion would always be 100% although the timing of the deaths could vary considerably.
The length of the period at risk must therefore always be reported along with the incidence proportion and taken into account in interpreting any reported value of incidence proportion.
In the previous example, 10 000 factory workers had normal hearing. A follow-up study of these 10 000 workers for the incidence of noise induced hearing loss over 10 years showed that a further 1813 workers developed noise induced hearing loss. The incidence proportion during the l0-year period therefore is
IP = 1813 / 10 000 = 0.1813 = 18%
