Qualitative exposure proxies

The accuracy of exposure assessment is determined by systematic and random errors in the assessment. For quantitative exposure assessments, important sources of error include measurement errors (i.e. from laboratory and field monitoring techniques), as well as variations in exposure over time and space. For qualitative exposure proxies (e.g. self-reported past exposures, occupational histories or expert evaluations), the most important sources of error are recall bias (systematic differences in exposure recall between cases and controls) and random error, expressed in terms of intra- and inter-rater agreement. Although systematic errors can result in serious misinterpretations of the data, especially due to scaling problems, random errors have received more attention in epidemiology because this type of error is pervasive, and its effect is usually to diminish estimates of association between exposure and disease. The magnitude of random errors can be considerable in epidemiological field studies. 

EXPOSURES RELEVANT TO HEALTH THE CONCEPTUAL FRAMEWORK 246 Considerations of Exposure Timing 246 Considerations of Exposure Route 247 Practical Context of Pesticide Exposure 248 EPIDEMIOLOGICAL STUDY DESIGNS AND EXPOSURE ASSESSMENT 248 Prospective Cohort Studies 248 Retrospective Cohort Studies 249 Case-Control Studies 250 Cross-Sectional Studies 252 EXPOSURE ASSESSMENT STRATEGIES 252 INFLUENCE OF THE ACCURACY OF EXPOSURE PROXIES ON MEASURES OF ASSOCIATION 254 Errors in Qualitative Proxies 254... 

First, this chapter will describe a conceptual framework to illustrate the special challenges posed because exposures assessed for epidemiologic studies must be relevant to the health outcome under investigation. Secondly, some of the most commonly applied epidemiological study designs will be introduced, with special emphasis on exposnre assessment issnes associated with the design. Thirdly, some widely applied exposure assessment approaches will be introduced, ranging from qualitative classifications of exposure to quantitative exposure assessment of pesticide concentrations. The influence of measurement error on measures of association between exposure and disease, such as the slopes of exposure-response relationships and risk or odds ratios, will be briefly reviewed. Finally, exposure proxies used in case-control studies of chronic effects of pesticide exposure will be reviewed and the concepts introduced earlier will be applied. 

Although it is difficult to generahze, the validity of qualitative proxies of exposure can be very poor in some cases. Tielemans et al. (1999a) recently compared... 

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