WPA

 In baseball, various batting metrics are used to evaluate hitters. Basic ones include batting average, slugging percentage, and BB/K ratio. There is also a metric called wRC+, which reflects a hitter’s pure offensive ability. If the hitter has high scores on these metrics, then the hitter will be evaluated as a good player.  

 However, to evaluate a hitter’s real ability in a clutch situation, we have to look at the hitter’s WPA(Win Probability Added). WPA quantifies the effect of each player's actions on their team's chances of winning, considering game circumstances like the score, inning, number of outs, and baserunners on the field.  

The formula for calculating WPA is as follows: WPA = WP_after − WP_before. If a hitter has a high WPA, it means that the hitter is making a significant contribution to the team's victory. For hitters with good overall stats but who don't feel impactful, looking at their WPA can help explain why. 

 Here is an example of a hitter in the KBO league, named Patrick Wisdom, who runs for the Kia Tigers. His wRC+ is 145.3, which is considered 'Great'. He also ranks second in the league with 22 home runs. However, there were even discussions about replacing him. What could be the problem with him. 

These are some stats from Widom’s 2025 KBO Percentile Rankings. We can see that his wRC+% and OPS(On-base Plus Slugging) are outstanding. However, his WPA stands at -0.517, which is a very low figure. In fact, a closer look at the detailed metrics shows that Wisdom is batting in the 0.100s in tie games or one-run situations, in the mid 0.200s when the score difference is within 2 to 4 runs, and nearly 0.300 when the margin is five runs or more. 

We can also compare WPA and player salaries. As of September 7, 2025, the WPA rankings for KBO hitters are Ahn Hyun-min, Yang Eui-ji, Park Min-woo, Koo Ja-wook, and Song Sung-moon. Ahn Hyun-min has a dominant WPA of 2.63, while the other four hitters record 2.19, 2.18, 2.17, and 2.17. Looking at their salaries, Ahn Hyun-min earns 33 million won, Yang Eui-ji earns 1.6 billion won, Park Min-woo earns 900 million won, Koo Ja-wook earns 2 billion won, and Song Sung-moon earns 300 million won. Considering that Ahn is still a rookie and Song recently signed a six-year, 12 billion won contract, we can see that WPA and salaries have a clear connection. The other three hitters are also among the highest-paid players on their teams. In particular, Koo Ja-wook ranks third in the entire KBO in salary. This shows that WPA can have a strong impact on determining player salaries. 

 WPA helps identify which players truly contribute to the team’s success. Based on this, managers and coaches can make more efficient decisions regarding lineup construction and pinch-hitting in certain chance situations. If it is further developed, WPA is expected to have a significant impact on professional baseball leagues in the future.