職業棒球運動首重球員表現，球團常藉頂級肥約來延攬球員，特別是明星打者。有鑑於此，近年來簽下長約的價值議題也更加備受關注。文獻上對於合約長度和球員表現關係亦未有明確定論。因此本研究系統化蒐集合約網站(Cot's Baseball Contract) 紀錄最早1996年至2013年的打者大量合約資料，利用詳細篩選過後的每一筆合約為樣本，以及常見的打擊表現指標，除了計算出球員在合約中的平均表現外，進而利用球員過去所有合約紀錄得出其生涯表現。藉由此方式控制各打者之間的先天能力差異做比較分析。此外，亦透過將合約做分類(短期、中期、長期)以及合約總值來探究對打者表現的影響。
本文實證結果顯示出生涯表現對於打者確實是一重要的影響變數，而長約對於打者不論是哪一打擊指標，結果幾乎多為負向顯著，可見不論導入生涯表現或僅是考量當下合約期間的平均表現，簽長約皆頃向會使得打者表現變得較差。若從合約的平均薪資上來看，考量生涯表現的情形下，平均薪資越高，對打者表現則未有顯著的激勵效果，結果不同於年齡對打擊表現的正向影響。團隊變數(如教練、團隊勝率等)則對打者個人的表現都沒有顯著的影響。
另一方面，在不同的合約分類中，簽長約相比中長約和短約來的對表現有更大的負向影響。再者，考慮整體合約總值，結果則為負向顯著，亦即合約薪資固定的情況下，合約長度相對薪資有較大影響力，球員可能會頃向降低薪資來換取更長期的合約保障。最後，基於本研究檢測，若球團想提供誘因留住球員又想規避長約的負面效果，則建議最多不要給出5年以上的合約。透過本研究，可提供球團在簽訂球員合約，以及進行球員交易時的參考。

The individual players’ performance is important in professional baseball. Also, the base- ball organization usually offer a fat contract in order to get potential players, especially for batters. In view of this, the issue of signing a long term contract has caught more atten- tion recently. However, There is no clear conclusion in the past literature. In this research, the contract data about Major League Baseball batters is collected on the Cot’s Baseball Contract website from 1996 to 2013. Through the detailed screening, we collect each obser- vation in the sample in terms of an individual contract and the common batting statistics to calculate the averaged performance in a certain contract and the lifetime performance up to starting a new contract. By controlling the batters’ innate ability, we can do some com- parison and analysis. Besides, we try to classify the contract sample into different duration and use total contract value in order to evaluate the batters’ performance.
The empirical results indicate that the lifetime performance is definitely a crucial factor for the batters. Moreover, the effect of contract length on batters’ performance is almost significantly negative. That is to say, signing a long term contract is worse for the players. Then, based on the empirical finding about average salary, there is no significant effect for batters while the more incentives are offered by the baseball team. By the way, the team characteristic variables like coach or team winning percentage is also no significant effect on the performance.
On the other hand, in comparison with short and medium term contract, we find the negative impact of long term contract on performance is the biggest. Next, the net effect of considering whole total contract value are also significantly negative. Perhaps, the players may lower the salary in order to get longer contract to extend his professional career. Finally, we do some test and suggest that the baseball team had better provide no more than five- year contract to a batter. The baseball team can use these results as a reference to evaluate the players.

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