What is the high outlier threshold?
There are two different quantities that sometimes are called high outlier thresholds. One is the statistical high-outlier test criterion or threshold computed by the Grubbs-Beck test and described on page 17 of Bulletin 17-B. The other threshold, which B-17 does not not clearly describe or distinguish from the statistical threshold, is the threshold that is used in (or implied by) the Bulletin-17-B historical adjustment procedure that actually is applied to the flood record. This threshold may be called the historical-adjustment threshold; it does not necessarily equal the statistical high outlier test criterion, and may be either higher or lower than the statistical test. The statistical high-outlier test criterion is based on the standardized distance, (x.i – x.bar)/stdv, based on logarithms of peak flows, between the extreme top observation and the mean of the data set. The criterion or threshold value is the value that is unlikely (10 percent chance) to be exceeded by the LARGEST o
