[A methodology to estimate incidence from prevalence rates in household surveys: the example of literacy].

Age and time-specific incidence rates are highly relevant to epidemiologists. Due to the mathematical relationship between incidence and prevalence, several methods have been developed to obtain incidence based on prevalence. The present study didactically describes a method to estimate age and time incidence, based on: repeated measurements of prevalence for an event; events related to the individual's age; and events with the characteristics of a chronic disease. The method was used to analyze literacy in the Brazilian population, using prevalence by age obtained from the National Household Sample Survey (PNAD) for 1981, 1985, 1989, 1992, 1995, and 2001, and allowed obtaining literacy incidence estimates for ages 5 to 20 years, for the years 1960 through 2001. The method adjusts incidence parameters for age and time with the help of a generalized linear model for categorical data and uses the EM algorithm to obtain non-observed data.