Incidence is the proportion of persons in a population who develop or demonstrate a particular disease or attribute during a specified period of time.

The numerator for incidence includes only those persons who develop the condition during the specified time period.

For example, in a study of 2390 women between 16 and 49 years of age, it was found that 482 used oral contraceptives. Twenty-seven of the oral contraceptive users developed bacteremia. The incidence was therefore: