from Amazon
Thursday, February 07, 2008
WPA for fielders
I get this question alot, so I might as well cement the response here:
How you wish to handle it is based on who is involved at the time the ball is in flight.
Let’s say that the win expectancy (WE) is .600 when the batter is at the plate. The pitcher makes a great throw causing the batter to pop high in the infield. 99% of the time, it’s a sure out, causing the WE to go down to .550. 1% of the time, it falls for a hit, for a WE of .700. The weighted average is .5515.
So, at the point of contact, you give the pitcher .600 minus .5515, or +.0485. Now, after the point of contact, the pitcher is out of the equation and the shortstop is now in the equation. If he catches the ball, the WE goes from .5515 to .5500. If he doesn’t it goes from .5515 to .7000.
It’s all a matter of tracking what the win expectancy is, at the point that the player is involved.