Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
only numerical errors but also formatting errors, identification
errors, etc., which can lead to a serious bias in the analysis of
data. There are several procedures for spotting such errors, from
simply reviewing the data visually to computerized procedures which
flag questionable data.
Finally, data sets should be validated, that is approved for
analysis. Data validation should form part of QA, and its procedures
should be clearly defined. Data validation usually consists of two
parts: data entry procedures to identify formatting errors at the time
of entry, and acceptance procedures which are designed to compare the
reported data against specified criteria in order to judge the
reasonableness of reported values. Acceptance procedures identify
various types of anomalies in the data including impossible values,
individual and multiple outliers and entire subsets of incorrect data.
11.4.3 Study plan and standard operating procedures
QA involves the development and use of study plans and SOPs. The
study plan (which should be very explicit and detailed) is the most
important document for providing information to all participants on
all aspects of the study, including information on responsible
personnel, sample collection, sample storage and pre-analytical
treatment, analytical procedures and data analysis. Deviations from
the study plan in, for example, sampling frequency, sampling media or
sampled population, necessitate a detailed amendment to the study
plan. This is very important since in many monitoring studies sampling
occurs on a regular basis, and the data obtained may be worthless if
the present sampling scheme is not followed.
SOPs are written instructions which describe how to perform all
field and laboratory activities. These should be available to all
personnel involved in the field and laboratory work and be a component
of their training programme. SOPs ensure that all personnel associated
with study operations will be familiar with and use the same
procedures. If different individuals perform important study
functions, such as storing samples, preparing solutions, running
analytical equipment, or archiving documents, these operations should
be performed in the same manner. By standardizing procedures for the
conduct of studies, SOPs have a valuable QA function. They prevent the
introduction of many potential errors in the generation, collection
and reporting of data.
The development of SOPs includes the following aspects: who
should prepare, review and authorize SOPs; which field and laboratory
operations require SOPs; and what information the SOPs should contain.
The nature of the laboratory work being done and the training and
experience of the laboratory personnel at a particular facility should
determine exactly how extensive the content needs to be (IPCS, 1992).
11.4.4 Collection of samples
The potential for contamination is a major issue in environmental
sampling. The highly sensitive analytical instrumentation currently
available permits analysis of very small analyte quantities where the
amounts of pollutants or chemicals present in environmental media are
often quite low (e.g., parts per billion). The risk of significant
contamination is particularly great when monitoring low concentrations
of substances ubiquitous in the environment or present in the
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