Coming off a season in which he posted a gruesome 5.18 ERA, James Shields is a favorite sleeper and undervalued asset of the statistically-inclined fantasy owner. The majority of these owners will cite Shields’ career high strikeout rate of 8.3 in 2010 as a primary reason to be bullish. In fact, I even mentioned it when I boldly predicted that Shields would post a sub-3.80 ERA in 2011. However, we are being slightly fooled by relying on that sneaky K/9 ratio.

In 2010, Shields’ 8.3 K/9 ranked 23rd among Major League starters with at least 90 innings pitched. However, using K/PA, he ranks a less impressive 36th. The problem with K/9 is that its denominator is based on outs. If a pitcher is having a poor outing in which he is struggling to record outs, that means he is facing more batters and has a greater opportunity for strikeouts. A pitcher’s BABIP will therefore have quite an impact on his K/9. Specifically, a pitcher with a BABIP well above the league average should be expected to post a higher K/9 than deserved. On the converse, pitchers with a low BABIP are likely posting an artificially lower K/9 because they are recording outs more frequently than they should be, giving them less of a chance at a strikeout.

The K/PA metric is not affected by that same problem. It does not care how weak or strong a pitcher’s defensive support is since it uses plate appearances in the denominator which are unrelated to outs. Though K/9 is obviously a lot easier to understand and find the calculation for, K/PA is clearly more useful.

Among the top 50 in K/9 last year (minimum 90 IP), the following table presents the top 10 pitchers with the largest difference between their K/9 and K/PA (K/9 ranks higher than K/PA):

And now for the top 10 whose K/9 ranks lower than K/PA:

Well this worked out even better than I could have expected. As the tables above show, the pitchers who rank much higher in K/9 than K/PA had both a BABIP and LOB% above the league average, and compared to the second group of pitchers, a higher HR/FB ratio. Though the pitchers in the second group are certainly not complaining about their results, their K/9 ratios understate their actual skills at striking out hitters, while the first group’s skills are overstated.

So how can we use this information for fantasy purposes? Assuming all else equal, better luck for the first group should cause a decline in those pitchers’ K/9 rates. However, with more neutral luck, their ERAs will drop as well, offsetting the fewer strikeouts and keeping their fantasy values stable. On the other hand, the second group of pitchers could see an uptick in their strikeout rates assuming their luck begins to dry up. This will possibly come with an increase in ERA though.

So to get back to the initial point about James Shields, yes, he was very unlucky last season and is an excellent rebound candidate. However, his career high strikeout rate was inflated due to the poor fortune he experienced and it should therefore not be leaned on to justify your optimism.