Are Matt Kemp's HRs less valuable?

I've seen a few comments on a couple different blogs where some people are under the impression that a fair number of Matt Kemp's home runs are hit in less meaningful situations.  For example, clubbing a homer when the team already leads by eight runs isn't really all that crucial, but hitting a two-run HR in the bottom of an extra-inning while trailing by one is huge.

I doubt I can come to a complete conclusion on how meaningful Matt Kemp's home runs are, because I either lack the time to collect enough data, or I just don't understand how to find the data I want quickly, but here is a little analysis that can at least push us towards a tentative statement on the effectiveness of his homers.

The fantastic provides easy access to a home run log for every player ever.  For example, here is Matt Kemp's home run log.  Notice that there is a column labeled "bWPA".  This stands for "Batting Team's Win Probability Added".  If you point at it, the pop-up balloon reads "Given average teams, this is the change in probability of the batting team winning the game from the start of this play to the end of the play."  For example, the first HR of Kemp's career changed the Dodgers win probability from 54% to 79% for a bWPA of 25%.  In essence, this is an attempt to measure how much this one swing of the bat affected this game.

I wish I could tell you what the average bWPA for a home run in the 2009 National League was, or for the 2009 Dodgers, but, alas, I couldn't figure out how to obtain that data.  But I did check the home run logs for the top five Dodger home run hitters of 2009 (Andre Ethier, Matt Kemp, Manny Ramirez, Casey Blake, James Loney) and the 2009 home run log for Albert Pujols, to get an idea of the kinds of numbers we might see.  Perhaps this is too simplistic, but I merely averaged the bWPA for each players 2009 home runs to see how much, on the average, that player's homers increased his team's chances of winning.  The table below show each player's bWPA per home run for 2009.

Player bWPA/HR
Ethier 17.3%
Kemp 12.4%
Manny 13.2%
Blake 18.6%
Loney 13.3%
Pujols 13.0%


Well, 12.4% is the lowest number in this group, but it is a lot closer to those three samples in the low 13% area than those 13%s are to the higher two samples.  Until we know what league averages are like, we can't make definitive conclusions, but I'll hazard a guess that Kemp's home runs are somewhere around typical in meaningfulness, on the average, and that Blake and Ethier are "clutchier" than typical, on the average.  And who would have guessed that Blake's average would be higher than Andre "Mr. Four Walk-off HRs" Ethier?

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