一般咸認打者面對使用與自身相異的慣用手投球的投手時，具有異手優勢。異手優勢屬於「事前」的範圍，而球員的表現則提供了「事後」的訊息，包括球員的技術、優/劣勢、以及球隊用人的策略等。就策略而言，球隊可能根據異手優勢安排不同選手的上場時間，進而影響到整個聯盟左手球員所佔的比例。本研究以西元2000年至2012年問美國大聯盟球員一百三十萬次的投打對決，提出異手優勢的量化分析及其對大聯盟左手球員比例的影響。結果顯示，異手優勢大約佔打者攻擊指數的7%至15%。其次，就上場時間而言，敵對左投手的比例對實力較差的打者影響較大，超過一般打者。最後，若純粹以球隊策略而言，大聯盟應該再多聘用7.5%的左打者。

It is generally believed that batters have an advantage when facing opposite-handed pitchers. While this opposite hand advantage is known to be ex ante, the performance of players is ex post, which contains information about their skill, hand (dis)advantage, and teams' recruiting strategies. Based on more than 1.3 million play-by-play data points in Major League Baseball from 2000 to 2012, we provide a quantitative estimate of the opposite hand advantage to show how it can be used as a determinant of the excess supply of left-handed players. The results indicate the opposite hand advantage accounts for about 7-15% of the ex post on-base plus slugging. Second, compared to average batters, marginal batters, both right-handed and left-handed, are more affected by the proportion of left-handed pitchers. Third, in terms of strategic advantage, 7.5% more left-handed batters should be employed in Major League.