Prevalence is a static concept that is a measure of state. It is a still-picture of the
disease situation at a given point in time. Whereas incidence relates to events, prevalence relates to disease states at a
point in time. The prevalence number is the number of cases of disease existing at the particular point in time. The prevalence
proportion = # cases of illness at a particular time / # of individuals in the population at the same time. Prevalence proportion is also called prevalence rate or point prevalence. Prevalence is not a rate but
a proportion; however the term prevalence rate has become so popular in medical literature that it will take long for this
error to be corrected. Prevalence is measured in cross-sectional studies. Only one observation at one point in time is needed
in the determination of prevalence.

PREVALENCE AS A PROPORTION

A proportion is the number of events expressed as a fraction of the total
population at risk without a time dimension. The formula of a proportion is a/(a+b) and the numerator is part of the denominator.
Like rates, proportions can be crude, specific, and standard. The variance of a sample proportion can be computed as {p(1-p)/n}
if N is large in relation to n. Sample proportion can alternatively be computed as [{p(1-p)/n}^{1/2}
{N-n}] / [N-1] If N is small in relation to n.

TYPES OF PREVALENCE MEASURES

Three types of prevalence are described in epidemiological literature: point and period
prevalence. Point prevalence is a theoretical concept that assumes ability to count cases of illness at an infinitesimal short
period of time. Period prevalence refers to counting the number of illnesses over a practically reasonable length of time.
This must not be so long that there is a change in the status quo by death of cases or incidence of new ones. Period prevalence
is more stable and therefore more useful than point prevalence.

RELATION OF PREVALENCE TO INCIDENCE

There is a relation among incidence, prevalence and duration. Prevalence proportion = incidence
rate x average duration of disease. Change in prevalence is due to: (a) Change in incidence (b) Change in duration: due to
dearth or recovery (c) Both change in IR and duration.

USES OF PREVALENCE

Prevalence is useful for administrative purposes. It is rarely used for etiological studies except for conditions in
which incidence is difficult to measure such as congenital malformations, non lethal degenerative diseases, and sero-conversion.
Prevalence is not good for etiological studies for the following reasons: (a) It can not distinguish the contribution of incidence
from that of disease duration (b) The time sequence is not obvious; disease and exposure are studied at the same time.