The predictive power of K:BB ratio: you'd be better off using a psychic

Ah, K:BB ratio, beloved metric of the ESPN fantasy baseball analyst.

By the end of last season, ESPN must've comment upon every pitcher's K:BB ratio in at least one fantasy baseball 'news' bulletin. The assumption appeared to be that a great K:BB ratio indicated that despite a bad ERA, better days were on the horizon (hello James Shields, Jason Hammel, Scott Baker et al), whilst a bad K:BB ratio but good ERA suggested luck and a forthcoming regression to the mean (hello Trevor Cahill).

Trouble is, K:BB ratio is not very predictive - i.e. it is easy to have a bad K:BB ratio and good ERA or a good K:BB ratio and bad ERA. This is quite evident from the list of last year's qualified starters ranked by K:BB ratio - in amongst the good pitchers like Roy Halladay and Cliff Lee, you'll see the undraftable likes of Doug Fister and Rick Porcello.









 Qualified Starters sorted by K:BB ratio

And here is the predictive value of K:BB ratio in graphical form (for the method behind the madness, click here):

An R^2 of 0.05 is rubbish, and means that K:BB ratio is less predictive of future performance than ERA - i.e. you'd be better off assuming that a pitcher with a high ERA will continue to have a high ERA and vice versa, ignoring the K:BB.

There are things that are better than ERA at predicting future performance, but K:BB ratio ain't one of them. Stay tuned to find out what they are.