Sample Rarefaction 

A randomized portion of the macroinvertebrates collected from a waterbody are identified in the laboratory. Complete identification of all individuals in a collected sample is neither cost-effective, nor necessary. Sorting and identifying a portion of the collected individuals drawn from uniformly distributed sample material accurately captures the richness and density of freshwater invertebrate communities. Subsequent to completing the taxonomic identification step, data are further subset (often called rarefaction) to ensure equal sample effort when comparing samples.

General NAMC benthic invertebrate identification protocol targets a final count of 600 total identified individuals. Samples from certain projects use 300 as the target count. Following identification, identified organisms are further randomly subsampled to a fixed count. The standard fixed count is 300 individuals per sample, although some indices have different fixed count targets.