The Biological Monitoring Working Party (BMWP) score system was introduced in 1980 to provide an index of river water quality for England and Wales based on aquatic macro-invertebrates. The score allocated to each taxon was set by a group of expert river biologists based upon their collective experience of the taxon's sensitivity to organic pollution (Hawkes, 1997). This paper describes an objective reappraisal of these subjectively-derived scores through the statistical analysis of a large and comprehensive database of field samples. Using the analytical approach of Walley & Hawkes (1996, 1997
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