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Doesn't your example above assume that all players get the same balance of clutch PA vs blowout PA though? I'm not quite understanding how the WPA - (WPA/LI) method guarnatees that a hitter who hits the same in all situations will get a clutch rating of zero, and my primary objection would be that this method doesn't directly, statistically, define what you're actually after.
WPA - Unleveraged WPA = the impact of having high leverage, not the batter's skill in high leverage.
You could bypass needing information about Ichiro as a hitter actually and still b edefining something real if you did:
SUM(WPA)/AVG(pLI) - SUM(WPA/pLI)...that expression, in English would be:
1) Find the player's total WPA
2) Adjust that total so that it reflects a neutral leverage (this is the element missing from the fangraphs method...there's no guarantee that the player hit in average leverage overall)
3) Subtract from his actual (neutral) WPA the unleveraged value of his plate appearances.
That gives you (directly) the number of wins your player added because of better than (his) normal performances in high leverage situations.
The AVG(pLI) tweak may not seem like a big deal, but it makes the thing logically consistent and it does have an impact..especially on teams that play a lot of close games.
Let's say that Ichiro, because he hits lead-off on a team that scores a lot of runs and gets into a lot of close games, gets 60% blowout PA and 40% clutch PA (this is an extreme example I know). Keeping the numbers from your example above, Ichiro would rate as +0.028 * 0.4 - 0.004 * 0.6 = +0.0088 Wins/PA. My method would correct for that by noting that Ichiro's net LI is not 1, but in fact 2.68 (I know this is very extreme...I'm doing this to prove a point). My way will give you zero clutch rating no matter what your LI distribution looks like if you hit the same in all situations.
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