Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
contains limits that are based on analytical standards, then the chart
is useful for discovering whether the observed measures for a sample
value differ from the adopted standard values by an amount greater
than should be expected by chance alone. Generally, control charts
consist of warning limits and action limits. Warning limits often
correspond to ± 2 SD from the mean, whereas action limits are set at
± 3 SD from the mean. Such thresholds mean that as long as the process
is in control at the centre value there is a 5% chance that a result
will exceed the warning limits and a 0.3% chance that a result will
exceed the action limit, thus erroneously signalling an "out of
control" message (WHO, 1986; ISO, 1993).
To set up a Shewhard control chart (Fig. 33), measurements of,
for instance, standard solutions, duplicates or spiked samples must be
gathered while the analytical procedure is in control. The control
parameters most frequently evaluated in Shewhard control charts
include the average or median of control measures (average or median
charts) and the range (difference) between duplicate analyses on the
same test sample (range charts) (UNEP/WHO, 1986; AOAC, 1991; ISO,
1991; Christensen et al., 1994). In a cumulative sum (cusum) chart
(Fig. 34) a reference value, usually the intended or expected value,
is subtracted from each observation. The cumulative sums of the
deviations from the reference value are formed, and these cusums are
plotted against the serial numbers of observations. The cusum chart is
usually more sensitive to small shifts in level than the Shewhard
chart and highlights persistent and recurring differences (Christensen
et al., 1994; ISO, 1997).
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