We would expect this temporal mismatch to introduce both Berkson, through the errors in the true exposures that were randomly selected within a month, and classical, through the errors in the temporal misalignment of the surrogate exposures, error components.
In most cases, the scans corrected exposure errors that resulted from my father's relying on a handwritten exposure table rather than a light meter to adjust his camera.
Below we quantify exposure errors that arise from variations in particle infiltration to aid evaluation of the use of this surrogate, rather than actual exposure, in PM epidemiology.
To assess the potential for spatially differential exposure error, we compared the spatial variability of exposure errors across ZIP codes.
Such exposure errors tend to reduce coefficients of response and their statistical significance.
In a multipollutant model, the absolute magnitude of this bias will depend on the variance of the exposure error, the correlation between exposure estimates, and the correlation between exposure errors.
Results: Pollutant concentrations and their exposure errors were moderately to highly correlated (typically, > 0.5), especially for CO, NOx, and EC (i.e., "local" pollutants); correlations differed across exposure metrics and types of exposure error.
The distribution of exposure error across ZIP codes was mostly negative (indicating that the exposure metric with a greater degree of refinement had a lower magnitude), although exposure errors were positive for a small number of ZIP codes.
Mean AORs in these simulations were 1.09 and 1.07, respectively, indicating greater sensitivity of the epidemiological results (the original AOR was 1.11) to independent exposure errors than to the shared exposure errors of the primary analysis shown in Table 2.
The correlation between the exposure estimates, the degree of exposure error for each pollutant, and the correlation of exposure errors between pollutants must all be considered in order to assess the impacts of exposure error on health risk estimates in a multipollutant model (Zeger et al. 2000; Zidek et al. 1996).
