Sub-index based on the observed and expected values of the SIGNAL biotic index and incorporates the tolerances of different macroinvertebrate families to common types of water pollution. SIGNAL (Stream Invertebtrate Grade Number Average Level) is a biotic index that uses a grade assigned to each family according to its sensitivity to pollution (Chessman 1995). A grade of 10 represents a high sensitivity to pollution, while a grade of 1 represents a high tolerance of pollution. The sum of the grades of all families present in a sample is divided by the number of families to give an average sensitivity (SIGNAL value) for the sample (Barmuta et al. 1998).