Posted by: Peter | April 10, 2010

Evaluating Liriano

The first few games of the season have been fun to watch, but the one I was really waiting for was last night’s start by Francisco Liriano. This was our first chance since last season to see him pitch in a meaningful game in front of the pitchf/x cameras, which means it’s our first chance to start evaluating all the winter’s hype. Is Liriano really back to something approaching his former greatness? Or are we in for another season of suckitude?

On the surface, the performance was decent but not great, with too many walks and too few strikeouts. But that could be a matter of luck or early-season jitters, and doesn’t definitively tell us what to expect from this point on. Digging a bit deeper into the numbers, I had three basic questions: how fast is Liriano’s fastball, how much break is there on his slider, and how well is he locating his pitches?

To get a handle on the first two, we can compare the speed and break of Liriano’s pitches this year to the last two years:

The vertical axis here is pitch velocity, in miles per hour.  The horizontal axis is “spin”, which is measure that uses some basic physics to approximate the amount of spin that the pitcher is putting on the ball. For more on precisely how it’s calculated, see here. Liriano’s three pitches form clear clusters: the fastballs on top, the changeups directly under the fastballs, and the sliders off to the right.

From this plot, it appears that last night, Liriano had a faster fastball, and more spin on his slider, compared to the past couple of years. This is definitely a good sign. To put the fastball in perspective, here’s what the average and standard deviation of Liriano’s fastball speed has been in each game going back to 2008:

On this graph, it looks like the improvement in velocity started at the end of last season. But those late-season appearances all came in relief, and most pitchers throw a bit harder coming out of the bullpen. If Liriano can keep this elevated velocity as a starter, it bodes well for him–as Mike Fast recently showed, increased fastball velocity generally does translate into fewer runs allowed.

Now let’s look at slider spin by game:

This is another good sign. Except for one game that was probably a fluke, this is the most bite Liriano has had on his slider in a long time.

Of course, a blazing fastball and a wicked slider aren’t that useful if you don’t know where they’re going. And clearly the biggest problem with Friday’s start was that Liriano threw too many balls. But ball-strike ratio is a somewhat crude measure of a pitcher’s ability to locate pitches. Sometimes, you want to throw a ball, and a sometimes a strike right down the middle will often lead to worse results than a pitch out of the zone.

Ideally, we’d have information about where the catcher sets his target, so that we could evaluate a pitcher’s results in relation to that.  We don’t have that information, unfortunately, so I thought I’d try another way of looking at location. The basic idea is to look at the percentage of a pitcher’s throws that result in “quality strikes”: balls that are around the edge of the strike zone, neither way outside nor right down the middle.

To determine what counts as a quality strike, I’ll use the strike zone model I described in this post. Basically, I assign each pitch a value between 0 and 1, indicating how likely that pitch is to be called a strike by the umpire. I call a pitch a “quality strike” if it has a more than 10% and less than 90% chance of being called a strike. This sounds like it would include a large area, but actually it’s just the area around the edges of the strike zone–the pitches in the middle of the zone have strike probabilities above 90%, while most pitches out of the zone are close to 0%.

Anyway, here is the percentage of quality strikes for Francisco Liriano–and, as a comparison, for Scott Baker.

player                  2008        2009       2010
Scott Baker             36.5        36.3       29.3
Francisco Liriano       29.4        32.6       38.0

In 2008 and 2009, Baker threw more quality strikes–which makes sense, since he was a better pitcher. But in 2010 (which includes only the first start for each pitcher), Liriano looks better than Baker (who struggled in his season debut) and better than he has in previous years.

This doesn’t mean Liriano is going to turn into an ace this year, and it’s clear that he really does have some control issues yet to work out. But all in all, I’m happy with his first start, and I think it was more encouraging than the numbers in the box score would suggest.


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