In epidemiology the denominator is key: who is “at risk” for a particular event or state.
RATE = Actual/Potential cases
- Rates are generally, but not always, determined per 100,000 by the Centers for Disease Control and Prevention (CDC).
Incidence and Prevalence
1. Incidence rate: the rate at which new events occur in a population. The numerator is the number of NEW events that occur in a deﬁned period; the denominator is the population at risk of experiencing this new event during the same period:
Number of new events in a speciﬁed period / Number of persons “exposed to risk” of becoming new cases during this period X10^n
Attack rate: a type of incidence rate in which the denominator is further reduced for some known exposure
Focus on acute conditions
all persons who experience an event in a population. The numerator is ALL individuals who have an attribute or disease at a particular point in time (or during a particular period of time); the denominator is the population at risk of having the attribute or disease at this point in time or midway through the period.
All cases of a disease at a given point/period / Total population “at risk” for being cases at a given point/ period
Point prevalence: prevalence at a speciﬁed point in time
Period prevalence: prevalence during a speciﬁed period or span of time
Understanding the relationship between incidence and prevalence:
a. Prevalence = Incidence x Duration (P = I x D)
b. “Prevalence pot”:
i. Incident cases or new cases are monitored over time.
ii. New cases join pre-existing cases to make up total prevalence,
iii. Prevalent cases leave the prevalence pot in one of two way: death or recovery
Morbidity rate: rate of disease in a population at risk; refers to both incident an prevalent cases