Epidemiological studies examine the incidence and prevalence rates of disorders in populations at risk and the factors associated with onset and recurrence.

**Measures of Disease Frequency**

Epidemiological studies examine the incidence and
prevalence rates of disorders in populations at risk and the factors associated
with onset and recurrence. A rate is determined by the number of cases (the
numerator) divided by the population at risk (the denominator).

Incidence rates refer to new cases that arise in a
healthy popula-tion during a fixed time. The most commonly applied incidence
rate in psychiatric epidemiology is the cumulative incidence rate, also known
as the “incidence proportion” (Selvin, 1991), that is, the proportion of a
population at risk that has a disease during a specified time. The range is
from 0 to 1. The numera-tor includes new cases of the illness, and the
denominator is composed of individuals at risk of becoming diseased for the first
time. In cumulative incidence, the duration of the observa-tion needs to be
defined (e.g., new cases in 1 month, 1 year, or 5 years).

Cumulative incidence is appropriate when a study
has a fixed cohort design (i.e., when all of the members of the cohort are
observed for the same time). However, when attrition oc-curs, the cumulative
incidence rate is a less desirable measure. Individuals lost to follow-up who
would have become case pa-tients are excluded from the numerator, whereas the
denomina-tor, which is the total population, remains unchanged. Moreover, those
who become lost to follow-up are often a biased subgroup of the original study
population (Eaton *et al.*, 1992).
Therefore, cumulative incidence should be considered most reliable when there
is a small loss to follow-up during the specified time. When loss to follow-up
occurs or when the occurrence of a health out-come is measured in a dynamic
cohort (i.e., when members of the cohort come in and out) different statistical
adjustments must be employed (Selvin, 1991).

Cumulative incidence = number of new cases /__ __total population
at risk

In epidemiology, incidence rate refers to the
number of new cases occurring in a specified time period divided by the sum of
time periods of the observation for all individuals in the population at risk,
or person-time (Rothman and Greenland, 1998). In effect, this statistic
measures the instantaneous force of morbidity or disease occurrence (Rothman
and Greenland, 1998). Miettinen (1985) also referred to it as incidence
density. The denominator person-time is the observational experience during
which a particular outcome may occur. The range of an incidence rate is 0 to
infinity. The incidence rate is measured in units of the reciprocal of time
(time21). The
units of incidence rates are time21. Because the units and the
numerical value of an incidence rate are difficult to interpret, incidence
rates are usu-ally compared with each other to obtain incidence rate ratios.

Incidence rate = number of new cases / person-years

Prevalence rates measure the proportion of
individuals who have the disease at a specified point or period in time.
Incidence re-fers only to new-onset cases, whereas prevalence includes all new,
recurrent, or chronic cases in the numerator and the entire population,
including those with a history of the disorder, in the denominator. The point
prevalence rate is the proportion of a population affected by a disease at a
given point in time. Period prevalence refers to the proportion of a population
affected by a disease during a specified time period, such as 6 months, 1 year,
or lifetime. Prevalence rates are influenced by the duration of a disease. For
nonchronic disorders, such as major depression, the point prevalence is usually
lower than the period prevalence. For chronic conditions, such as
schizophrenia, the point prevalence and period prevalence are expected to be
similar.

Prevalence rate = number of cases/ total population
at risk

In general, prevalence data are less useful than
incidence data for etiological research. Prevalence is determined not just by
factors that cause a disease but also by factors secondary to the disease
itself. On the other hand, prevalence measures are use-ful in public health or
service utilization situations. For instance, the geographical location and
planning of specific services of a from prevalence studies.

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Essentials of Psychiatry: Psychiatric Epidemiology : Measures of Disease Frequency |

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