This issue has been puzzling me for a while, and the Westbrook signing really brought it to the forefront. Given Westbrook's method of pitching, is it fair to exact him to the same evaluation methods that we apply other more conventional hurlers to? First, let's examine Jake's peripherals, which are usually the most useful in making predictions.
Strikeouts per 9 innings: 4.8
For the average pitcher, this isn't really an encouraging sign. The ability to strike out hitters is the surest way of run prevention, and pitchers with lower strikeout ratios have a lower margin for error in allowing runs.
Walks per 9 innings: 2.6
This is pretty good. Westbrook finished 14th in the AL in BB/9, which doesn't really signify anything by itself.
Hits per 9 innings: 8.7
Above average numbers here, but nothing to get excited over.
Home Runs per 9 innings: 0.79
This is pretty good. Jacobs Field has been a fairly neutral park in recent seasons, so I'll just assume that this ratio doesn't need to be adjusted much.
DIPS ERA: 4.19
DIPS stands for Defense Independent Defense Statistics. This is, essentially, Westbrook's ERA independent of his defense. Given that his ERA was .90 runs lower than his dERA, this is a red flag. This seems to indicate Westbrook was lucky last season.
BABIP stands for "Batting Average of Balls in Play". This is the "batting average" of balls put in play. And like the dERA, the BABIP indicates Westbrook was lucky with where where his pitches were hit.
SLG% Against: .386
This is an encouraging sign. This is the slugging percentage of the batters that faced him last season, and it's the 4th best in the league. This indicates the hits Westbrook did allow were more likely to be singles than extra-base hits.
GB/FB Ratio: 2.72
Westbrook only trailed Derek Lowe in this category.
What does all this indicate? First of all, Westbrook was pretty lucky last season, and his ERA is likely to go up. But I'm wondering out loud if Westbrook's sinker causes more "weak contact" than the average pitcher. I watched most of his starts in 2004, and when he was on, hitters couldn't hit the ball with any authority, and most of the time any contact resulted in weak grounders to short or second. DIPS ERA uses the assumption that MLB pitchers do not differ greatly on their ability to prevent hits on balls in play, but there are a couple possible exceptions. One is a knuckball pitcher, whose general goal is to keep hitters off balance. Another one could be extreme groundball pitchers, who try to pitch to "weak contact" by throwing heavy sinkers. I don't know of any way to substantiate this, so for now this is a just a wild theory. I'll peg Westbrook for an ERA of 4.25 or so next season based on his peripherals, but I'll also be interested to see if his BABIP changes appreciably. Obviously to be successful with a strikeout rate of 5.0 or less, a pitcher has to keep his walks down as well as keep the running game under control, both of which Jake did in 2004. But if his BABIP continues to stay low, it may indicate that his sinker is good enough almost by itself to make Westbrook a valuable pitcher.
Sources: ESPN, STATS, Inc.