The collection sample should contain 100+ arthropods. A tolerance value of 0 to 10 is assigned to each arthropod species (or genera) based on its known prevalence in stream habitats with varying states of detritus contamination. A highly tolerant species would receive a value of 10, while a species collected only in unaltered streams with high water quality would receive a value of 0. The sum products of the number of individuals in each species (or genera) multiplied by the tolerance of the species is divided by the total number of specimens in the sample to determine the HBI value.
where n = number of specimens in taxa; a = tolerance value of taxa; N = total number of specimens in the sample.
Precautions should be taken to account for confounding variables, such as the effects of dominant species over-abundance, seasonal temperature stress, and water currents. Limiting the collection of individuals from each species to a maximum of 10 (10-Max BI) has been shown to minimize the effects of these phenomena on the True BI.
The biotic index is then ranked for water quality and degree of organic pollution, as follows: