"Plus" stats compare a player's stats to the league average. Since baseball statistics have varied greatly across baseball history, Plus stats provide a useful way of comparing performances in different seasons.
If you are unfamiliar with "Plus" stats, this page explain the basics of Plus stats. The example here will use home runs, but the same concept applies to any baseball statistic.
For our example, let's look at 2 Yankee third basemen who led the AL in HRs:
- In 1976 Graig Nettles led the league with 32 HRs. He had 583 ABs, giving him a HR/AB ratio of 5.49%.
- In 2007 Alex Rodriguez led the league with 54 HRs. Coincidentally he also had 583 ABs, giving him a HR/AB ratio of 9.26%.
- In 1976, the AL league average HR/AB ratio was .0171 (1.71%).
- In 2007, the AL league average HR/AB ratio was .0288 (2.88%).
- The league average expected HRs is calculated by multiplying the league average against his ABs. So 583 ABs * .0171 = 9.9693.
- His HR+ is calculated by dividing 32 by 9.9693, then multiplying by 100, then rounding, resulting in an HR+ of 321.
- League average expected HRs: 583 * .0288= 16.7904
- HR+: 54/16.7904 * 100 = 322
If you would like more information on Plus stats, Diamond Mind Baseball provides a more detailed explanation.
Finally, it's worth mentioning that Plus stats can be applied to careers, as described on this blog's Adjusting Career Home Runs Allowed Using HR-Plus page.