(a) by aggregating the most recent
Industrial Product Price Index for Motor Gasoline
or the most recent Industrial Product Price Index for Diesel Oil, as the case may be, fo
r each month in the period, including the relevant data for the period from January 1, 1981 to December 31, 1985, released by Statistics Canada under the authority of the Statistics Act on or before the fifteenth day of the third month f
ollowing t ...[+++]he end of that period and adjusted or altered in the manner prescribed pursuant to subsection (3);