[Proportional mortality studies: criteria for the selection of participant groups]

Proportional mortality designs are used widespread in occupational epidemiology. In this review those biases which can affect them, mainly the healthy worker bias, are discussed. Several options for their analysis and the assumptions to be accomplished for validity are reviewed: proportionate mortality analysis, standardized mortality ratio, and odds ratio. The inclusion of dead participants in a research exhibits several drawbacks. Starting out from the analysis of this sort of designs, the first criterium to select diseases is similar to case-control studies: the reference group must not include diseases related with the exposure under study. Analyzing the relationship between mortality and incidence rates, criteria to select diseases to be investigated by proportional mortality studies are offered. These designs yield a valid inference when the disease is rare and irreversible. If the exposure shortens duration of disease, a toward-the-null bias is introduced. The direction of bias is variable under other circumstances, although it shows a trend to be negative.