Adjusting Baseball Statistics Using Plus Stats

"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%.

Looking at only the raw statistics, one would say that A-Rod was nearly twice as powerful as Nettles. However, this would ignore the reality that in the 1970s, home runs were much harder to hit than in the recent era. For example:

• 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%).

Comparing stats compiled in different seasons is where Plus statistics are useful. A Plus statistic divides the actual number of batting events to the expected number based on the league average, then multiplies the quotient by 100, then rounds to a whole number. Let's show how the Home Runs Plus (HR+) statistic is calculated for Nettles' 1976 HRs:

• 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.

Now let's do the same for A-Rod's 2007:

• League average expected HRs: 583 * .0288= 16.7904
• HR+: 54/16.7904 * 100 = 322

Their HR+ stats are virtually identical. In other words, HR+ tells us that both A-Rod and Nettles were both around 320% above league average in hitting home runs in these seasons. In other words, you could say that hitting 32 HRs in 1976 is comparable to hitting 54 HRs in 2007.

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.