What's Behind Jake Arrieta's Fast Start?

Third year Orioles starting pitcher Jake Arrieta is off to a very nice start to the season through three starts with a 2.66 ERA and 0.89 WHIP.  Buck gave him the nod on opening day and so far has been rewarded for showing faith in the young right hander.  The ERA estimators agree that his good start is legitimate rather than just a result of good luck.  Currently, Arrieta has a 3.33 FIP, a 3.55 xFIP, and a 3.55 SIERA.  All of these are much improved marks from anything that he's put up in his previous two seasons.  Has Arrieta changed at all as a pitcher?  What's behind this good start?

Since three starts is still only about 10% of his work for the season, we have to be careful to not read too much into certain numbers.  Most of his statistics are not reliable at this point.  I'll just get it out of the way now...small sample size.  I know, I know.  But if we didn't analyze baseball in small sample sizes at this point in the season, what would we talk about?  With that out of the way, I'll do my best to only look at the stats that become at least semi-reliable in the small sample sizes.

The first stat that becomes reliable at this point in the year is velocity.  And that just so happens to be much improved for Jake so far this year.  Check out this velocity graph from fangraphs.  Click on it to embiggen.

His average fastball velocity so far this season has been 93.8 mph.  In 2010 and 2011, it was around 92.5 mph.  So through his first three starts, he's gained about 1.5 mph on his fastball.  There is no question that is a positive development.  I don't recall the exact number, but someone did a study on the effect of an increase in velocity for a starting pitcher.  There was something like a .50 ERA improvement for every mph increase.

It's more informative to look at his K% and BB% rather than K/9 and BB/9 at this point in the season, since it makes the sample size bigger.  Usually, I don't mind using K/9 and BB/9 but this early in the season, the percentages are better.  Through 79 batters faced, Arrieta has a 20.3 K% which is up from 17.8% last year.  So he's made a small improvement in his K%. 

Drilling down even farther, Jake has a swinging strike % of 7.5% so far in 2012.  In 2011, it was just about the same at 7.4%.  It's been shown that swinging strike % has a high correlation with K%.  League average has been about 8.5% for the last few years, so he's actually below average there.  He's accumulated 16 strikeouts in his 79 batters faced and 20.1 innings thrown.

It's in his walk rate where he's made a big improvement.  He's only walked 4 of the 79 batters he's faced this year, which is a rate of only 5.1%.  If he could somehow keep this BB%, we will see a big step forward out of Arrieta this year.  There's no way to really know if he can keep this up.  Throughout his minor league and major league career so far, his walk rate has been around 11%.  Hopefully, this is real improvement but his BB% just hasn't stabilized yet so it's hard to say.

The last thing I'll look at are his luck stats.  His BABIP is an extremely low .214.  Obviously, this won't last and some of those line drives he's been giving up will start falling for hits.  Jake is stranding 67.9% of runners that reach base, which is slightly below the league average of 72.5%.  We can expect this number to improve slightly.  Lastly, his HR/FB rate is fairly normal at 9.1%.

What I'd take away from Arrieta's performance so far is the fact that his velocity is up 1.5 mph, he's getting a little lucky on his balls in play, and he has greatly improved his walk rate.  If he can maintain the velocity and keep some or most of the gain to his walk rate, I think we're going to see the best year of Jake Arrieta's career this season.  Maybe the Orioles did have a real opening day starter after all.

I'll leave you with this look at the pitch f/x graph of his pitches so far in 2012. Enjoy.