Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
The Shewhard chart has six control limit lines corresponding to
±rho p, ±2rho p, and ±3rho p (rho p = rho y / n 1/2 ) (rho p, appointed
standard deviation in the control chart; rho y, estimated standard
deviation of the distribution of results; n, number of control
results) (Christensen et al., 1994).
11.5.3 External quality control
The aim of external QC is to demonstrate that analytical results
obtained are accurate and comparable with the results ascertained by
other laboratories. Usually, external QC measurements form part of
external QA schemes or proficiency testing programmes. In such a
scheme or programme, the coordinating laboratory prepares a
homogeneous reference sample and distributes portions to the
participating laboratories for analysis. Participating laboratories
are required to examine the reference samples within a specified time,
preferably together with collected samples, and submit the results of
the reference samples to the coordinating laboratory which collates
the data, performs a statistical analysis and sends an evaluation
report back to the participating laboratories. In cases of poor
performance, the laboratories may be contacted, and suggestions may
made for improving performance.
External QC schemes offer participating laboratories many
advantages. They demonstrate the reliability of laboratory results and
provide independent evidence of the quality of laboratory performance
and individual analyst proficiency. In addition, external quality
schemes allow participating laboratories to compare their own
performance with that of other laboratories. Participation in an
external QC scheme can encourage self-appraisal and minimization of
laboratory errors. It can also be used to reduce the frequency of
internal QC efforts when consistently favourable results are achieved
on external QC test samples. Furthermore, it can assist in identifying
needs for training and changes in laboratory procedures (AOAC, 1991).
Examples of existing external QC schemes are given in Table 37
(see also Christensen et al. (1994)).
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