Two common practices of occupational mortality studies have no model in experimentation: (a) the use of the "general population" as a reference population and (b) the use of the total number of deaths as a surrogate for the population-time of follow-up. The former tends not to secure validity in terms of (i) comparability of effects, ie, identity of the extraneous effects of the compared experiences; (ii) comparability of populations, ie, absence of intractable confounding; and (iii) comparability of information, ie, identity of the certification of deaths from the illness of interest for the contrasted populations. The use of a carefully selected, occupational reference population is necessary for all three types of comparability. When deaths from other (auxiliary) diseases are used to estimate the relative magnitudes of the compared populations, careful selectivity is again called for. With respect to auxiliary causes of death, also, the compared occupational populations must satisfy all three aspects of comparability, with the added requirement that the exposure under study have no effect. The "healthy worker effect" is the result of failure to use comparable reference populations in occupational mortality studies.