The numerator is the number of new cases in a particular population during a specified time period.
How does one identify all new cases in a population during a specified time period? There are two possible approaches:
- Monitor the entire population repeatedly over time
- Screen the entire population for diseaseà follow those without disease for a specified time periodà rescreen for diseaseà identify new cases, follow up non-diseased for specified time periodàrescreen for disease….
In the second approach, any cases diagnosed from those previously non-diseased at initial screening, must be new cases.
Numerators alone are not very informative, as the denominator is required to see the numerator in perspective. However, if the denominator is stable over time, one may be able to draw useful information from a scrutiny of the numerator alone. For all practical purposes, however, both numerator and denominator must be available to make valid comparisons.