As the Australian Baseball League 2011 season wraps up, I thought it made sense to come up with a pitching metric to evaluate pitchers pitching in the league. Since we didn't come up with a reliable A and AA pitching metric, I filled out the rest of the spreadsheet with minor league pitchers (from the White Sox system) who had pitched at all 3 levels.
The ABL players that have pitched in A-ball had an average ERA of 3.39 in the ABL. Those same pitchers have a 3.51 ERA in A-ball, a difference of .12. Those pitchers averaged a 1.19 WHIP in A-ball and 1.16 in the ABL, a .03 difference. So there is a slight upgrade from the ABL to A-ball, but the difference is very small, so no one will judge you if you just use the A-ball metric we are about to create and use it on ABL players.
The pitchers on the list that pitched in AA had a 3.16 ERA and a 1.21 WHIP in A, and a 3.7 ERA and 1.32 WHIP in AA (a .54 ERA difference and .11 WHIP difference). The pitchers that pitched in AAA had a 3.43 ERA and 1.233 WHIP in AA, and 4.32 ERA and 1.37 WHIP (a .83 ERA difference and .137 WHIP difference).