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.