美國職棒大聯盟實施分區賽制後，至今已超過50年的時間。分區內的對手除了對戰次數多之外，所有球隊也得共同競爭分區的冠軍，才有機會拿到季後賽的席次。其制度對於球隊戰績的影響也經常成為人們討論的話題。像是今年，大聯盟大量減少同分區比賽後，可以看到分區之間的戰績相差更加懸殊。

　　在產業組織的研究上，許多結果皆證明競爭情況改變會使廠商的產能、利潤、創新等表現受到影響。而這是否也會發生在大聯盟的球隊上面呢？本文想透過雙重差分法(difference-in-differences)進行實證，利用2013大聯盟太空人隊的分區遷移案，分析其中部分分區的球隊數量增減是否真的對於該分區的球隊的戰績造成影響。

　　我們的實證最後說明，受到太空人隊遷移案影響的分區，無論是面對競爭者減少或是競爭者增加的情況，勝率都顯著的變得更好。除此之外，本文也發現球隊所處的分區其他球隊的實力越強，該球隊與分區外其他對手交手的勝率也會顯著性的提升。分區賽制在不少層面上恐怕有著不小的影響力。

　　It has been more than 50 years since Major League Baseball implemented the division institution. In addition to having more games with other teams in the same division, all the teams in the division must compete for the championship of the division to get a place in the postseason. The impact of division on the team's standings has often discussed by fans. For example, this year, after the major leagues greatly reduced the number of games in the same division, the standings difference between the divisions is becoming larger than last year.

　　In the study of industrial organization, many scholars have proved that changes in competition affect the performance of manufacturers such as productivity, profits, and innovation. Will this also happen to teams of MLB? Through difference-in-differences (DID) as our analytical approach, using the 2013 major league Astros divisions realignment case as empirical case, to find whether the increase or decrease in the number of teams in some divisions influences the standings of their teams.

　　The result of our research concludes that teams affected by the divisions realignment have significantly better winning percentages whether their divisions have fewer or more competitors. In addition, we also find that the stronger the other teams in the division where the team is located, the greater the team's winning percentage against opponents outside its division. The division institution may have considerable influence on many aspects.

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