Is There a Correlation Between Interleague Winning Percentages and DH/P/PH Offensive Production?

(The following is being syndicated from The Captain’s Blog).

Heading into the second half of the interleague schedule, the American League holds a 66-60 advantage over the National League. If that winning percentage holds, it will continue the senior circuit’s gradual improvement since losing over 60% of interleague contests in 2006.

Historical Interleague Records, by League

Note: Data as of June 23, 2011
Source: MLB.com

Most experts agree that the American League’s recent dominance of interleague matchups has been the result of having better players and stronger teams. Less clear, however, is whether one league or another enjoys an inherent interleague advantage, all else being equal.

For a recent article, I compiled aggregate data for all interleague-related plate appearances, which were defined as follows: DHs hitting in an AL park; pitchers hitting in a NL park; and pinch hitters batting in the ninth slot in a NL park. Not surprisingly, American League DHs posted an OPS that was 0.084 points higher, while National League pitchers outperformed by 0.070 OPS points.  What was interesting, however, is that when all of the relevant at bats were totaled, the combined performance was nearly identical. In other words, it seems as if neither league enjoys an overall advantage during interleague play.

Even though the aggregate league totals don’t suggest an advantage, what about individual teams? In order to answer that question, the aforementioned interleague-related plate appearances were compiled on an team-by-team basis using game logs. Then, the combined output for each ballclub’s offense was compared to totals given up by its pitching staff.  Finally, each team’s relative performance was compared to their interleague record.

Despite using this more granular analysis, the same general trends were observed. All but three American League teams (Orioles, Royals and Tigers) enjoyed greater production from the DH slot, while all National League teams underperformed from that position. On the opposite side of the ledger, only three teams from the senior circuit (Reds, Marlins and Pirates) had lower output from the pitcher’s spot in the batting order, while two junior circuit teams (Angels and Indians) managed to wrestle away an advantage. The real question, however, is what conclusion, if any, can be drawn from the aggregated data?

Net OPS in “Interleague PAs” (bars) versus Winning Percentage (line)

PH portion of “interleague PAs” based on all pinch hitters used to replace a batter hitting in the ninth slot. Pinch hitters used for pitchers batting in other slots have been omitted, and pinch hitters replacing a ninth place batter who is not the pitcher have been included.
Note: Data as of June 23, 2011
Source: Baseball-reference.com

As it turns out, there seems to be a significant correlation (R=0.70) between “net interleague output” and winning percentage in those games. In particular, the relationship seems strongest at the extremes. Among the seven teams with an OPS edge of more than 50 points, only one doesn’t have a winning record (not surprisingly, the Cubs are the team that defies success). Meanwhile, all six teams with an OPS deficit of at least 50 points have a losing interleague record.

Although the correlation between interleague production and winning percentage is a strong one, that doesn’t mean pitchers, DHs and pinch hitters solely determine the outcome in these games. There are many more reasons why the Pirates have the worst interleague record and the Yankees have the best, but it does seem as if the team that accentuates its league-based advantages starts off with at least a small head start.