Improving trends estimates for specific work-related ill-health conditions when excess zeros are present in a voluntary health reporting scheme.

Background: Trends in occupational disease incidence are estimated in voluntary reporting schemes such as The Occupational Health Reporting Network (THOR) in the UK. Voluntary reporting schemes request responses even if no cases are observed to reduce uncertainty in non-response. This may result in false zeros that bias trends estimates. Analysis using zero-inflated models are unsuitable for specific health outcomes due to overestimates of the excess zeros. Here we attempt to account for excess zeros whilst investigating condition specific trends.

Methods: Zero Inflated Negative Binomial (ZINB) models were fitted to three THOR work related ill health surveillance schemes EPIDERM (437 reporters between 1996-2019), OPRA (1094 between 1996-2019), and SWORD (878 between 1999-2019). The probability associated a response being a false zero were estimated and applied in weighted Negative Binomial (wgt-NB) models fitted to specific ill-heath conditions. Three ill-health conditions from the three THOR schemes were considered; contact dermatitis, musculoskeletal, and asthma respectively.

Results: Wgt-NB models approximately estimated the incidence rate ratios reported by the ZINB models (e.g. EPIDERM;ZINB=0.969,NB=0.963,wgt-NB=0.968) for all health outcome annual trends. This was consistent for specific health outcomes which also tended towards the null (e.g. Contact Dermatitis;NB=0.964,wgt-NB=0.969) indicating potentially overestimated downward trends. Though as the ratio of excess zeros to true zeros decreased in rarer health-outcomes, the influence on trends also decreased.

Conclusions: Through weighting we were able to adjust for excess zeros in health outcome specific trends estimates. Though uncertainty is still present in underlying reporter behaviour meaning caution should be applied with interpretation of any results.