[Establishing endemic levels of ranges with computer spreadsheets].

Epidemics or disease outbreaks can be defined as an excess in the number of cases of a given health problem, in a particular population, period, and place. However, to determine what constitutes an excess implies knowing what is normal or to be expected. Establishing endemic levels or ranges makes it possible to determine figures for the number of expected cases and to show in graphic form the appearance of a higher number of cases. In this work a new methodology is described to develop these endemic ranges, in which the Quattro Pro (Qpro) and Excel software programs are used to calculate the geometric mean of historical rates and their confidence intervals. Also presented is a procedure for cumulative endemic ranges that facilitates surveillance of endemic events with a low incidence.