Bulletin of the World Health Organization

Correcting for numerator/denominator bias when assessing changing inequalities in occupational class mortality, Australia 1981–2002

Gail M Williams, Jake M Najman, & Alexandra Clavarino



Comparisons of the changing patterns of inequalities in occupational mortality provide one way to monitor the achievement of equity goals. However, previous comparisons have not corrected for numerator/denominator bias, which is a consequence of the different ways in which occupational details are recorded on death certificates and on census forms. The objective of this study was to measure the impact of this bias on mortality rates and ratios over time.


Using data provided by the Australian Bureau of Statistics, we examined the evidence for bias over the period 1981–2002, and used imputation methods to adjust for this bias. We compared unadjusted with imputed rates of mortality for manual/non-manual workers.


Unadjusted data indicate increasing inequality in the age-adjusted rates of mortality for manual/non-manual workers during 1981–2002. Imputed data suggest that there have been modest fluctuations in the ratios of mortality for manual/non-manual workers during this time, but with evidence that inequalities have increased only in recent years and are now at historic highs.


We found that imputation for missing data leads to changes in estimates of inequalities related to social class in mortality for some years but not for others. Occupational class comparisons should be imputed or otherwise adjusted for missing data on census or death certificates.