The age-standardized mortality ratio (SMR) is a relative index of mortality, expressing the mortality experience of the study population relative to that of a comparison ("standard") population. With the general population as the "standard", the SMR for an occupational population will underestimate the mortality experience of that latter population (since it comprises individuals necessarily healthy enough to be employable --and whose mortality risk is therefore initially lower than the general population average). However, this "healthy worker effect" does not equally to all groups within the study population. Therefore, if one attempts to adjust for this effect, the summary nature of the SMR must be recognized, and allowance must be made for variation in the healthy worker effect between different age groups, different races, different work-status groups, different causes of death, and different elapsed-time periods of observation.