Last update:
06-Jun-2005

Epidemiology is the scientific branch that studies the frequency of occurrence of diseases (health states or death), as well as the characteristics of individuals on which this frequency depends. Measures of occurrence, for the quantification of morbidity, are distinguished into empirical and theoretical. This division is based on the domain, which for the empirical measures of occurrence is population-time or a series of person-moments, while for the theoretical measures it is an abstract category. Empirical measures of occurrence are divided into measures of incidence and measures of prevalence. Empirical measures of incidence are separated into measures of incidenceproportion and measures of incidence-density. Incidence-proportion is the percentage of person-moments, from the beginning of follow-up (T₀=scientific time), developing the disease during a period of time. Instead of dividing the number of cases of disease by the number of person-moments, the number of cases is divided by a measure of time. This time measure is called population-time and is the summarization, across all individuals, of the time experienced by the population being followed. Both incidence-proportion and incidence-density are measures that assess the frequency of disease onset. The numerator of either measure is the frequency of events that are defined as the occurrence of disease. In contrast, prevalence does not measure disease onset but is a measure of disease status. Prevalence is the percentage of the population that has the disease at a certain point in time. A theoretical measure of occurrence cannot calculate, but has a priori a specific but unknown value. This value is estimated by use of empirical measures of occurrence that are realised in a certain domain. Theoretical measures of occurrence are incidence-proportion, prevalence-proportion, risk and odds. Risk is defined as the probability of an individual developing an undesirable event (disease onset or death) during a certain period of time.

Key words: Incidence-density, Incidence-proportion, Measures of occurrence, Prevalence, Risk.