DEFINITION OF PREVALENCE

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.