   Last update: 18-Dec-2014 Arch Hellen Med, 31(6), November-December 2014, 735-747 APPLIED MEDICAL RESEARCH Measures of frequency in epidemiological studies P. Galanis Center for Health Services Management and Evaluation, Department of Nursing, National and Kapodistrian University of Athens, Athens, Greece

In epidemiological studies, measures of frequency are used for the measurement of the frequency of the occurrence of the outcomes, while measures of association are used for the estimation of the relation between determinants and frequency of the occurrence of the outcomes. Measures of frequency are empirical and theoretical. In epidemiological studies, empirical measures of frequency can be calculated, while theoretical measures of frequency cannot be calculated but are estimated with the empirical measures of frequency. Empirical measures of frequency are measures of incidence and measures of prevalence. Empirical measures of incidence include incidence-proportion and incidence-density, while measures of prevalence include point prevalence and period prevalence. Outcomes in measures of incidence are events, while in measures of prevalence are states. Theoretical measures of frequency are incidence-proportion, incidence-density, point prevalence, period prevalence, risk and odds. The value of a theoretical measure of frequency cannot be calculated but has a priori a specific and unknown value that is estimated with the respective empirical measure of frequency in a specific study population. At first, the determinant, the outcome and the study population are defined and then the appropriate measures of frequency are calculated. In closed populations, incidence-proportion, incidence-density and prevalence are calculated, while risk is estimated with incidence-proportion. In open populations, incidence-density and prevalence are calculated. When the outcome under study is the death and the study population consists of healthy people, then the incidence of the death is called mortality. In closed populations, mortality is calculated as incidence-proportion or incidence-density, while in open populations, mortality is calculated as incidence-density. When the study population is closed and consists of patients with a specific disease and the outcome under study is the death from this disease, then incidence-proportion of death or case fatality rate is calculated.

Key words: Case fatality rate, Incidence-density, Incidence-proportion, Measures of frequency, Mortality, Prevalence.