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3.3.2 Conversion of

adjusted predictions to

reference values

This step is to convert the adjusted prediction from a
value representing the High/Good boundary to a value
representing the WFD reference condition – ie to a WFD
reference value (Figure 3.16). The WFD reference value of
an index is defined as its median value at WFD reference
sites (sites that comply with WFD reference criteria) and it
is somewhere in High status. The conversion factor is the
ratio of the median value of the index across all RIVPACS
reference sites (both GB and NI models) to the median of
the value from RIVPACS reference sites that are also in
WFD reference condition.
Ideally, we would have derived separate conversion
factors for each end group, but that was not possible
because, in some end groups, none of the reference sites
were in reference condition. The effect of conversion is
relatively small, but it does ensure that, as a whole, the
UK’s river invertebrate classification is equivalent to those
used in other countries.
The adjustment is described in a report by Clarke and
Davy-Bowker J (2006). (49) This report can be downloaded
from the SNIFFER website https://www.sniffer.org.uk/ Figure 3.17
(search for WFD72b). Front cover of Clarke & Davy-Bowker,
SNIFFER project WFD72b (49)
RICT2 can only predict reference values for the few
indices used for the UK’s general degradation status
assessment. To help derive adjustment and conversion
parameters, Clarke & Davy-Bowker also produced an
adjustment calculator in Excel and a Minitab macro called
FitM4. The spreadsheet can be downloaded from the
same web address as the report. The Minitab macro
enables the Excel prediction adjustment spreadsheet to
be used to adjust any biotic index or metric by calculating
new coefficients for it. The original Minitab Macro was
based on RIVPACS III, which has a different number of
end groups, so it has been modified for RIVPACS IV. The
modified macros and instructions for using it are available
from John Murray-Bligh.

Paraleptophlebia submarginata

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