Back to our original discussion on the effects of foul area (AF) on strikeouts (SO). The following post is based on an abstract submitted and accepted for presentation at SABR 42 in June in Minneapolis. It would have marked the thrid consecutive year StrikeThree had an abstract presented at the SABR meeting, had I been able to arrange my schedule to attend. This seems to indicate there is considerable interest on the subject of foul area and strikeouts, even though the quantitative relationship still escapes confirmation.

The NL has historically SO more hitters than the AL. I've pointed out that the only consecutive years where that hasn't been the case since WWII is 1964-68, the only 5 consecutive years that the AL has led the NL, while the Indians were setting AL SO records! The effect of AF on SO has likely had an impact on the differences in SO between the AL and the NL as well.

From 1947-53 and from 1954-63, NL parks averaged 29,400 sq. ft. foul area, and AL parks 32,000 sq. ft.. This 2,600 sq. ft. difference translates to 8.9% more average AF in the AL compared to the NL, adjusted for 198 ball-park years. Less AF, fewer foul balls caught on field, more strikeout opportunities and strikeouts, as we've pointed out in the earlier posts.

The NL led the AL in SO / 9 innings from 1954-63 by 3.9%, and by approximately 4.5% from 1947-1963. Smaller, lower strike zone in NL? Different chest protectors for umpires and different umpire vantage point for calling pitches? Perhaps. But smaller AF likely contributed to a higher-SO NL as well!

From 1964-68, NL AF decreased even further to 27,100 sq. ft., while the AL decreased to 31,400 sq. ft. However, despite the increased difference in AF to 4,300 sq. ft. , the AL now led the NL for all 5 years, as the Indians led the rest of the AL by 21% more strikeouts. It appears the Tribe staff dominance was sufficient to not only obscure, but also reverse, any tendencies of low strike zones, different umpire equipment and positioning, differences in AF, or other unnamed factors, to lead to more SO in the NL. The data is summarized in Table 1.

Analysis of the difference in later years needs yet to be performed. Specific examples of later changes in ballpark foul area affecting SO might be seen in the differences between yearly SO curve between the AL and NL. Convergence of the historically separate SO curves of the NL and AL has been identified in the early 90’s, after Comiskey II (’91) and Jacobs Fields (’94) were introduced, as might be expected if 2 of the larger stadiums (Cleveland Municipal and Comiskey) were replaced by smaller stadiums (Figure 1). However, 2 of the larger AF in the NL (Mile High and Fulton County) were subsequently replaced by 2 smaller (Turner and Coors) shortly thereafter. This then allowed subsequent divergence of curves even beyond usual historical trends, with continued parallel between SO in the AL and NL according to the previous years.

Figure 1. SO/ 9 Inningsings in the AL and NL, 1947 to 2009. Increase SO in the AL 91-94 when Cleveland Municipal and Comiskey were replaced, narrowed the gap between the leagues:smaller foul area, more AL SO! The gap immediately widened again as Coors and Turner, both with lower AF, were introduced: smaller foul area, more NL SO! Note also that the long-term historical advantage of the NL is broken only from 1964-68.