In the case of BMWP/Organic pollution rankings, the presence of mayflies or stoneflies for instance indicate the cleanest waterways and are given a tolerance score of 10.
The number of different macroinvertebrates is also an important factor, because a better quality water is assumed to contain fewer pollutants that would exclude "sensitive" species - resulting in a higher diversity.
Kick sampling, where a net is placed downstream from the sampler and the river bed is agitated with the foot for a given period of time (the standard is 3 minutes), is employed.
Any macroinvertebrates caught in the net are stored and preserved with an alcohol solution, and identified to the family level, this can be done with the live organisms as well.
Other indices that can be used to assess water quality are the Chandler Score, the Trent Biotic Index and the Rapid Bioassessment Protocols.