8.3. The performance and efficiency factor for each group of municipalities constituted under section 8 is determined by carrying out the following operations in the following order:
(1) once the performance and efficiency factor for each municipality in a group has been determined under section 8.2, the 2 subsets formed by the factors situated, respectively, in the lowest 12.5% and the highest 12.5% are excluded, and the arithmetic mean of the factors remaining between those 2 subsets is then calculated;
(2) the standard deviation is calculated, that is, the mean difference between the remaining factors mentioned in paragraph 1 and the arithmetic mean established under that paragraph; and
(3) the results obtained in paragraphs 1 and 2 are added together.