Effect of sampling on measurement errors

Abstract

Often the analyst is taken as a guarantor for data quality in spite of the fact that sampling is commonly performed by others. If the analyst ignores sampling uncertainties, the money spent on quality control of analysis may sometimes be in vain. The analyst ought to be aware of the difference between controlling exposure and measuring workers' exposure at the workplace. When controlling exposure the aim is to ensure that workers' exposures are below the given occupational exposure limits (OELs); when measuring exposure the aim is to determine what the worker is actually exposed to, on average. In the working environment, exposure is usually controlled by measuring "worst case' situations, i.e., situations where exposure is higher than average by an unknown amount. As pointed out by Eisenhart (cf. Anal. Chem., 1981, 53, 1588A), measuring without a state of statistical control being attained cannot in any logical sense be regarded as measuring anything at all. Except for substances for which the OELs are ceiling limits that must not be exceeded, 'worst case' results cannot be used for documenting non-compliance or for risk assessment, epidemiology or standard setting. Measuring workers' exposure requires estimation of the time weighted average concentration in the exposure period considered (TWAC exposure Period) by carrying out measurements, preferably over a series of days (TWAC Day). Kromhout et al. (Ann. Occup. Hyg., 1993, 37, 253) found TWAC day data to be lognormally distributed with a median geometric standard deviation of 2.5. Sampling from such distributions is shown to give very disperse results. Consequently, many measurement days are needed. A TWAC Exposure Period estimate, therefore, is either very uncertain or has been very costly to obtain. In order to obtain more reliable results at an affordable cost, an alternative approach, called the logbook method, has recently been suggested for the estimation of TWAC Exposure Period. Commonly, workers considered to be similarly exposed are grouped. In contrast, the logbook method groups processes causing similar exposures. The time component of exposure is measured by workers keeping logs of their activities over a period of several weeks.