Category Archives: USMLE

incidence & prevalence

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

Prevalence rate:

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


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