EFFECT OF SAMPLING ON MEASUREMENT ERRORS

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 analys t ignores sampling uncertainties, the money spent on quality control o f analysis may sometimes be in vain, The analyst ought to be aware of the difference between controlling exposure and measuring workers' exp osure at the workplace, When controlling exposure the aim is to ensure that workers' exposures are below the given occupational exposure lim its (OELs); when measuring exposure the aim is to determine what the w orker 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 am ount, 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-comp liance or for risk assessment, epidemiology or standard setting, Measu ring workers' exposure requires estimation of the time weighted averag e concentration in the exposure period considered (TWAC(Exposure Perio d)) by carrying out measurements, preferably over a series of days (TW AC(Day)). Kromhout et al, (Ann. Occup, Hyg,, 1993, 37, 253) found TWAC (Day) data to be lognormally distributed with a median geometric stand ard deviation of 2.5, Sampling from such distributions is shown to giv e very disperse results, Consequently, many measurement days are neede d, A TWAC(Exposure Period) estimate, therefore, is either very uncerta in or has been very costly to obtain, In order to obtain more reliable results at an affordable cost, an alternative approach, called the lo gbook method, has recently been suggested for the estimation of TWAC(E xposure Period) Commonly, workers considered to be similarly exposed a re grouped, In contrast, the logbook method groups processes causing s imilar exposures, The time component of exposure is measured by worker s keeping logs of their activities over a period of several weeks.