text: To quantitatively assess changes in the composition of biologic communities, IBIs are developed to accurately reflect the ecological complexity from statistical analysis. There is no one universal IBI, and developing metrics that consistently give accurate assessment of the monitored population requires rigorous testing to confirm its validity for a given subject.  Often IBIs are region-specific and require experienced professionals to provide sufficient quality data to correctly assess a score.  Because communities naturally vary as do samples collected from a larger population, identifying robust statistics with acceptable variance is an area of active and important research.
  This can be a powerful tool to identify systemic impacts on the health of biological systems.  IBIs are increasingly involved in the identification of impairment, and confirmation of recovery of impaired waters, in the total maximum daily load process required by the Clean Water Act in the USA.  
  Unlike chemical testing of water samples, which gives brief snap-shots of chemical concentrations, an IBI captures an integrated net impact on a biological community structure.  While the complete absence, particularly sudden disappearance of, suites of indicator species can constitute powerful evidence of a specific pollutant or stress factor, IBIs generally do not resolve a specific cause of impairment.
  The IBI concept was formulated by James Karr in 1981. To date IBIs have been developed for fish, algae, macroinvertebrates, pupal exuvia (shed skins of chironomidae), vascular plants, and combinations of these.  Comparatively little work has been done to develop IBIs for terrestrial ecosystems.