Published 29 April 2009, doi:10.1136/bmj.b1695
Cite this as: BMJ 2009;338:b1695

Endgames

Statistical question

Incidence and prevalence

John Fletcher, clinical epidemiologist

1 BMJ, London WC1H 9JR

jfletcher{at}bmj.com

The average survival after a diagnosis of multiple sclerosis is much longer than after a diagnosis of cervical cancer. They both have about the same incidence of five per 100 000 per year, although this varies with geography and demography. In a large population the prevalence of multiple sclerosis would then be expected to be:

a) The same as that of cervical cancer
b) Much less than that of cervical cancer
c) Much greater than that of cervical cancer
d) Less than the incidence of multiple sclerosis
e) Greater than the incidence of multiple sclerosis

Show Answers

Answer

c—Incidence is measured in units of number of cases in a population in a time period. Prevalence has no units and is simply a proportion—the number of existing cases in a population at a given time. Because prevalence and incidence are different and measured with different units it makes little sense to compare their numerical values as in d and e.

In a stable population with stable disease characteristics prevalence is equal to disease incidence multiplied by average duration of disease. Because people live much longer on average after a diagnosis of multiple sclerosis than after a diagnosis of cervical cancer, the prevalence of multiple sclerosis is much greater than that of cervical cancer.

Cite this as: BMJ 2009;338:b1695


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