prevalence are closely related. Prevalence (the proportion of
a population with a problem at a designated time) depends on both
the incidence (the rate of new problem during a period of time)
and the duration of the problem as is illustrated by the following
4.1: Illustration of Incidence and Prevalence
with a long duration will have a high probability of being encountered
at that designated time when prevalence is estimated.
the prevalence of chronic disease in a general population is growing
because of both the higher incidence of chronic diseases (there
are more cases each year than in the past) and the longer survival
of patients with chronic disease resulting from modern treatments.
If death rates drop, then the time that patients live with chronic
disease grows and even a low incidence will produce a high prevalence.
assumptions, the inter-relationship between incidence and prevalence
can be expressed by this formula: Prevalence = incidence x average
is often used to describe the number of new cases of a particular
disease, for example, lung cancer, breast cancer, heart disease.
Incidence is useful in understanding how commonly a disease or
problem occurs over a period of time, which can be used to estimate
one's risk of getting the disease.
is a more relevant measure than incidence when assessing the impact
of a problem within a community and to assess the subsequent needs.
Although data on prevalence are easier to collect than incidence
data, prevalence should be used with caution in evaluating associations
between variables. In studies of prevalence it is very difficult
to distinguish the effects of factors affecting the occurrence
of disease from the effects of factors that increase survival
(or duration of the problem). This difficulty often leads to incorrect
imagine you are planning health services in a community and you
want to know how many services are need to care for patients with
motor neuron disease. The incidence is relatively low. But if
only incidence was used to determine service needs, the need might
be underestimated. The mean survival for patients with motor neuron
disease is three to five years. During many of these years patients
experience increasing symptoms and problems. Thus prevalence is
more useful to assess the need for services.