Anatomy of ERA

take a look at these two lines for two houston astros SPs:

Pitcher A:  54.7 IP  54 H  14 BB  50 K  6 HR  3.50 DIPS
Pitcher B:  55.0 IP  58 H  16 BB  27 K  5 HR  4.34 DIPS

the difference in ERA for these 2 starters is a full run. for those of us who subscribe to voros mccracken's DIPS theorey, it would be expected that pitcher A has the lower ERA. that is not the case, though. pitcher A, wandy rodriguez, has an ERA of 4.45 while pitcher B, chris sampson, has an era of 3.44.

the main difference between these two pitchers' statistics is that wandy rodriguez has struck out 23 more batters than sampson. everything else is remarkably similar. according to DIPS, rodriguez's 23 extra strikeouts give him an advantage of 8 tenths of a run in expected ERA. what factor could possibly be significant enough to account for the 1.8 run difference between expected DIPS era and actual ERA?

the answer is not as sexy as one might think. it all comes down to the difference between pitching with the bases empty versus pitching with runners on. here are the numbers:

Chris Sampson:
bases empty:  133 PA  .296/.338/.488  12 XBH  5 HR
men on:        99 PA  .247/.320/.329   6 XBH  0 HR

Wandy Rodriguez
bases empty:  133 PA  .262/.301/.421  12 XBH  3 HR
men on:        94 PA  .256/.326/.476  11 XBH  3 HR

it really is that simple. 5 more XBH and 3 more HR with men on base led to 8 more runs scored on rodriguez than were scored on sampson in this situation.

i think this is a trend that warrants tracking through the rest of the season. i've seen it exhibited by relief pitchers such as geoff geary, and have had this to say in the past about it:

"to guess why there is such a profound difference between geary's performance with runners on base and with the bases empty, i would say that with the bases empty, geary elevates his fastball earlier in the count, leading to more solid contact against. without looking at video to confirm, i can only guess that geary's front side flies open in the windup, leading to the higher pitch location."

- steagles
-april 20, 2007

i have yet to follow up on geary's situation, but i will keep an eye on both he and sampson to see what similarities they posess.