The biotic index of the arthropod fauna of streams is modified by limiting to ten the number of individuals in each taxon used in its calculation. This reduces detrimental effects on the index of certain fairly tolerant taxa in clean streams, effects of a few tolerant taxa in somewhat polluted streams, and the effect of our inability to identify larvae in some insect genera to species. It also greatly reduces seasonal variability, allowing use of the biotic index throughout the year with only a minimal decrease in the sensitivity of the index during the summer months. The EPT index was highly variable and exhibited seasonal variation in most of the streams.
Hilsenhoff, William L.
"A Modification of the Biotic Index of Organic Stream Pollution to Remedy Problems and Permit Its Use Throughout the Year,"
The Great Lakes Entomologist, vol 31
Available at: https://scholar.valpo.edu/tgle/vol31/iss1/1