The LBGR is needed to calculate the number of data points, N, which is used to test whether the survey unit concentration is less than the DCGLW. In order to calculate the shift, there must be an LBGR value (shift = DCGLW – LBGR). The shift (or width of the gray region) is then used to calculate the relative shift, which is an intermediate step necessary to calculate N, where N is the arithmetic standard deviation of the measurements. The relative shift expressed in multiples of standard deviations.
The gray region is a range of values for the parameter of interest for a survey unit in which the consequences of making a decision error are relatively minor. The upper bound of the gray region in MARSSIM is set equal to the DCGLW, and the lower bound of the gray region (LBGR) is a site-specific variable.
For more information on the MARSSIM process, visit MARSSIM on EPA.gov.