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CHAPTER 2 11 11

LABORATORY VERSUS FIELD ANALYSIS

OF INVERTEBRATE SAMPLES

Laboratory analysis includes
sieving, sub-sampling, sorting,
and identification of animals.

Poor weather and light will affect biological data recorded
from field sorting. The main benefit of laboratory analysis is
that its errors are well understood and have been quantified.
Because of that, samples to be analysed by RIVPACS,
particularly those used for WFD status classification, must
be sorted and identified in the laboratory under controlled
conditions, not in the field.

Measures of laboratory error for laboratory-analysed
samples based on independent audit are incorporated
in RICT (River Invertebrate Classification Tool). Audits of
laboratory analyses of invertebrate samples by regulatory
agencies in Great Britain also provide estimates of bias (the
impact of non-random error on biotic indices, see Section 13)
and quantitative information about error. Estimates of bias are
incorporated in WFD status classification so that they can be
accounted for in estimates of probability of class, and results
can be adjusted to take account of variations in analytical
quality.

Analysis in the field is less accurate and less precise. It is Field analysis is most effective when it is used with data
not suitable for WFD status classification because far more analyses that are optimised for field data. These improve
precision is needed to differentiate good from moderate efficiency and limit errors by concentrating on key taxa
status reliably than is possible with field analysis. However, and features that are suitable for field analysis (see, for
field analysis is ideal where high precision is not needed, example, Chapter 5 Section 3.1.26 – Rapid Appraisal
such as rapid screening for gross pollution. It also allows Key for detecting farm pollution). These methods tend to
the proportion of living and dead animals to be recorded, be accurate but less precise than laboratory methods.
which can be important evidence for assessing the impact of Biotic indices used for status classification, such as WHPT
pollution. Because of that, samples collected for investigating (Whalley Hawkes Paisley Trigg), are not designed for use
major pollution incidents are often analysed in the field and with field data and they include taxa that cannot be identified
again in the laboratory. reliably in the field.

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