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Archive for September 13th, 2011

The Red Sox’ bandwagon has slowly begun to empty as the team struggles through a nightmarish stretch during which it has lost six games to Tampa in the standings.  Fortunately, those abandoning the Boston ship have found passage on the Tigers’ pennant drive. Over the last three weeks, Detroit has won 16 of 20 games and built an insurmountable lead in the Central.

If the season ended today, the surging Tigers would meet the slumping Red Sox in the ALDS. Judging by the trending conventional wisdom, this clash of teams going in opposite directions would have Red Sox Nation tuning into the Patriots a lot earlier than expected, but should we rely on September records when handicapping post season series in October?

In a 2009 Baseball Prospectus post, Jay Jaffe looked at winning percentages over defined periods in September and found no meaningful correlation to various measures of post season success. However, the study did not distinguish between the relative records of opponents, nor any relationship between overall winning percentage and the rates compiled over the defined periods. In addition, Jaffe’s study was limited to post season series played since 1995. So, as a supplement to his work, the following incorporates the two aforemetioned variables into a breakdown of every first round post season series since 1969.

First Round Post Season Outcomes, Based on Relative Opponent Records (click to enlarge)

Note: Includes all first round post season series since 1969. Ranges refer to the winning percentage disparity between teams. A negative range indicates that a team with a lower record won the series.
Source: Baseball-reference.com and proprietary calculations

There have been 116 first round post season series since 1969 (LCS from 1969 to 1993 and LDS in 1981 as well as from 1995 to 2010). Of that total, 65 series (or 56%) were won by teams with the better regular season record, while in 58 (or 50%), the victor was the team with a better winning percentage in the final month. According to this data, the hottest team’s chances of winning in October were no better than a coin flip. However, if we assume those teams tend to be inferior (i.e., they have to win just to make the playoffs, while better teams have the luxury of resting their players down the stretch), then perhaps a 50/50 chance actually represents an enhancement?

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