Sunday, May 11, 2008

Why strikeouts matter for Zach Duke

Today Zach Duke and Jair Jurjjens take the mound with the Pirates looking for their sixth straight win and second straight sweep, this time over a team much better than the lowly Giants. With Duke on the mound, this is as good a time as any to take some space and explain why I'm so worried about his ghastly low strikeout rates this year.

The reason for my concern over Duke's strikeout rates is two-fold. First off, the Pirates' defense is bad. Their defensive efficiency, a raw measure of balls put into play that they turn into outs, is .680, second worst in the league. There's a drop-down menu to allow you to look at the efficiency ratings from the past and the Pirates are always towards the bottom of the league with a rating somewhere between .675 and .680. So we know that the Pirates are terrible at turning batted balls into outs. The more balls Duke allows in play, the more hits and runs he's going to allow. That's just how it goes with the Pirates.

The main reason, however, is that for Duke, strikeout rate seems to be an indicator of how well he's pitching. In general, it's accepted that pitchers don't have a lot to do with the outcome of a play once the ball leaves the bat, assuming it stays in the park. Defensive positioning, defensive ability, etc., all play a role on hits made off of even the best pitches. There are, however, some exceptions. Sinker-ballers, for example, usually have low strikeout rates but can have good years because of the huge numbers of ground balls they induce. On the other hand, a really bad pitcher that gives up a ton of hard hit balls is going to have a higher percentage of balls put into play turned into hits. In Duke's first season with the Pirates, he struck out 6.16 batters per 9 innings and batters hit .303 against him on balls they put in play (that's to say at-bats that ended in something other than a walk, strikeout, or home run). Given the Pirates bad defense, .303 is probably about what should be expected, since .290 is generally the accepted average. In 2006, Duke's strikeouts fell to 4.9/9 and his BABIP(batting average on balls in play) against rose to .336. So not only was he striking out fewer hitters, more of the balls opponents put in play were going for hits. In 2007, his K/9 plummeted to 3.4 and his BABIP against rocketed to .374. You can see that in his career, Duke's strikeouts and how hard the opponents are hitting the ball against him are inversely proportional. That's why I use strikeouts as a shorthand for how well he's pitching: because in his career the numbers have generally indicated that when he's pitching well, he's striking batters out and when he's not pitching well, he's not striking batters out. This year he's striking out 2.7/9. That's why I'm not encouraged by his decent start.