A measure of change from non-disease to disease (which is the numerator) in a "population-at-risk" (which is the denominator) over a specific time period.
By "population-at-risk," we mean all persons in the population who have not been diagnosed with the disease of interest at the beginning of the observation period, but who are capable of developing the disease
Newly diagnosed cases / population at risk
Describes the occurrence of new disease in the population
A static measure of the proportion of a population that is diseased, whether the disease cases occurred recently or at some time in the past.
Prevalence measures reflect already existing disease.
Existing cases/ total population