Methodological approaches in occupational epidemiology

K2.1 PERSON-TIME AT RISK: UNIFYING CONCEPTS FROM AN OLD IDEA

D. Loomis.Department of Epidemiology, University of Carolina at Chapel Hill, USA

The analysis of person-time risk data from working populations dates from the earliest years of quantitative epidemiology: William Farr and other pioneering statisticians had begun to use census and death registration data to generate occupational mortality rates over 150 years ago. Initially person-time at risk was approximated by the number of people living in a given year (usually tabulated according to demographic categories defined by sex and age). Today we understand data of this type to estimate the rate of incidence, a fundamental epidemiological indicator of disease occurrence.

Although the incidence rate has natural and historical connections to the analysis of population health, modern epidemiology has tended to emphasise study designs—such as the clinical trial and the case control study—that treat time at risk in different, and less natural, ways. The parallel development of statistical methods has also favoured techniques adapted to these designs, notably the logistic and proportional hazards regression models. While these developments have undoubtedly advanced the state of the art, they have done so at a price. The challenges of articulating explanations of “odds” and “hazard” might be mentioned, but for occupational epidemiology the most significant concern is lack of clarity about the time related nature of exposure.

Occupational health researchers are usually interested in exposures whose level can change over time. When exposure changes in this manner, the risk of disease may also vary continuously in time as a function of current and previous exposure. However, common epidemiological designs and analytical techniques typically do not take this temporal variability explicitly into account. One side effect of this methodology is that the relevant exposure may be obscured, making it difficult for …