The Chi2 test is used to determine if a test site falls within the experience of the AUSRIVAS model. Any test sites with no appropriate reference group for comparison are identified as being 'outside the experience of the model'

AUSRIVAS determines that a site is outside the experience of the model if its critical value is within the 0.1 percent area (alpha = 0.001) in the tail of the Chi2 distribution.

In the case of a site being outside the model’s experience, the site is colour coded in red, the error message detailed below is shown and no output is produced for that site.
 

 

AUSRIVAS also determines when a site is nearly outside the experience of the model. In this case, the critical value falls within the 1 percent area (alpha = 0.01) in the tail of the distribution.

If a site is nearly outside the experience of the model as determined by the above test, the site is colour coded in orange and a warning message is given (shown below). In this case, output data for the site in question is still produced.
 

 

Further information

Further information can be obtained from the paper:

Derivation of a biological quality index for river sites: comparison of the observed with expected fauna. R.T. Clarke, M.T. Furse, J.F. Wright & D. Moss. Journal of Applied statistics, Vol 23, No's 2 & 3, 1996, pp311-32.