Incidence rate

When you calculate incidence risk, you assume that the whole population studied is simultaneously at risk from the same starting date, and at the end of the period, say a year, a few of the population will have developed the disease.

That's why the denominator includes those people who eventually get the disease. This is in fact the prevalence at the end of the specified period.

This works well for large populations when you can estimate fairly accurately the population at risk at the beginning of the year.

But with smaller populations you have to allow for the fact that the individuals don't start and finish being at risk at the same time.

So we have to measure each individual exposure and add them all together to quantify the total period at risk by the whole population.