Incidence is the number of new events arising within a specified
time period. The word incidence usually denotes incidence rate.
Incidence rate is the rate at which new events occur in
a population. The numerator is the number of new events arising in a specified
time period; the denominator is the population at risk of an event during
this time period, sometimes expressed as person-time.
Observed annual incidence of x is therefore the number of
new cases of x diagnosed in a given population over a year divided by
the number of the population at risk of x during that year.
The use
of person-time as opposed to just "time" enables you to handle
situations where there are drop-outs in a study or where you have not
been able to follow an entire cohort at risk to watch for the event under
investigation. Using person-time calculations, the follow up period does
not have to be the same for all individuals studied. Person-time for a
group is the sum of the times of follow up for each participant in that
group. See person-time incidence
rate comparisons and Poisson rate confidence interval for more information.
Prevalence is the number of
events in a population at a designated time. The word prevalence usually
denotes point prevalence ratio.
Point prevalence is the number of persons with a disease
or attribute at a point in time. Point prevalence ratio is expressed as
below.
Period prevalence is the number of persons who have had the disease or attribute at any time over a specified period. Period prevalence is often expressed as below but it is not strictly a ratio it is a rate.
Prevalence ratio is equal to the incidence rate multiplied by the duration of the event.