Cohort studies measuring risk (incidence proportion)
Cohort studies that measure risk compare occurrence of disease between exposed and unexposed cohorts. The risk (incidence proportion) of disease in those exposed (IPe) and unexposed (IPu) can be computed as follows:
In the above example IPe = Ce/Ne
In the above example IPu = Cu/Nu
The absolute effect of exposure on disease occurrence is the risk difference (RD) between the exposed and unexposed cohorts.
The relative effect of the exposure on disease occurrence can be expressed as the risk difference between exposed an unexposed, divided by (relative to) the risk in unexposed.
Where RR is the risk ratio defined as:
Cases of gastroenteritis according to consumption of food X, nursing home A
One can express the result by saying that the relative effect of consuming food X is 3 which would suggest a 300% increased risk of gastroenteritis among exposed. One can also express the results by saying that the risk of disease is 4 times higher in the exposed cohort than in the unexposed cohort.
Thus the relative effect is the risk ratio minus 1. Since the relative effect is RR - 1, epidemiologists frequently refer to RR as a measure of relative effect without subtracting 1. The term "relative risk" is very popular among epidemiologists even if, as mentioned above, it is not a measure of relative effect but rather a risk ratio. When using the relative risk that way we have to remember that a value of 1 corresponds to an absence of effect.