What’s up with Jake Arrieta? He began the year with improved velocity and through May 2nd he had two outings where he gave up 0 runs and only one disastrous outing (a 4 inning, 5 run outing in LA against the Angels). However, since May 2nd Arrieta appears to have turned into a pumpkin. On May 8th against the Rangers Arrieta gave up 6 runs in 6 and 1/3 innings and yesterday as the O’s appeared poised for a sweep of a hobbled Tampa Bay team, Arrieta was chased in 3 and 2/3 innings after giving up 7 earned runs.
It’s undebatable, right? Arrieta was good, then something happened and now he isn’t good anymore. However, Arrieta’s skills (strikeout rate and walk rate) in his two bad starts appear strong. The factors that are causing Arrieta to look inept (high batting average on balls in play, a high home run to fly ball rate and a low strand rate) are typically considered “random” or “luck-based”. Here we’ll look at Arrieta’s skills (strikeout rate and walk rate) in more detail and explain why Arrieta doesn’t have as much control as you may think he does over the three factors that have doomed his last two starts. Analysis after the jump, all graphs in the posts are courtesy of our friends at Fangraphs.
There are two outcomes of an at bat that a pitcher is directly responsible for walks (BB) and strikeouts (K). That is it. Major-league pitchers have a lot less control than we think they do over which: (1) balls in play become hits and (2) fly balls become home runs. Throughout 2012 Arrietta’s skills, his walk rate and strikeout rate, have been pretty stable. In fact, Arrieta’s strikeout rate has been improving. The two graphs below show each skillsthroughout each start of Arrieta’s career. It is important to note that currently Arrieta is posting career high marks in strikeout rate per 9 innings and a career low walk rate per 9 innings.
As we discussed earlier, Arrieta does not have a lot of control over the three factors that have plagued his last two starts. These factors are a high batting average on balls in play (high BABIP), a high home run to fly ball rate (high HR/FB) and a low strand rate (low LOB %). Most relievers and nearly every starting pitcher strand about 72% of the runners that they put on base. In other words, only about 28% of base runners ever score. So, when a pitcher has a season with a high LOB% (76-99%) they are usually lucky. If they have a low LOB% (0-68%) then they have been unlucky. Jake Arrieta’s current LOB % is a 60.6% and it has plummeted over his last two starts. Arrieta’s LOB % throughout his career is shown in the graph below.
In addition to dealing with variations in LOB%, pitchers may also have bad luck in the form of BABIP variations. BABIP, or batting average on balls in play, tends to stay very near .300 for pitchers. When it strays much higher than .300, a pitcher is usually unlucky. Likewise, as a BABIP approaches .000, a pitcher is usually increasingly lucky. In Arrieta’s two starts his BABIP has sky-rocketed. Arrieta’s BABIP throughout his career is shown in the graph below.
Readers should expect Arrieta’s LOB % to be ~72.0% throughout the remander of the season and Arrieta’s BABIP to be ~.300. These regressions to the mean will address two of the three factors for Arrieta’s current struggles.
The final factor haunting Arrieta is the rate at which the fly balls he is giving up are becoming home runs. A bevy of work has shown that pitcher home run rates are extremely variable over time. A pitcher may allow home runs on only 5% of their flyballs one year (Clay Bucholz in 2010), yet then turn around and allow 11% the next year (Clay Bucholz in 2011). Due to this variability, home runs rates on flyball are considered fairly random. In 2010 only 7.0% of Arrieta’s fly balls became home runs, in 2011 this figured more than doubled to 15.0% for Arrieta. Currently in 2012, 14.0% of Jake’s fly balls are leaving the stadium and in the last two games 23% of Arrieta’s fly balls are leaving the yard. This is unsustainably bad. In the future, readers should expect Jake’s FB/HR ratio to regress to the league average (~10.0%).
Metrics that predict future ERA believe that in the remainder of the season Jake’s ERA will lie between 3.50 and 4.00. Jake’s skills have progressed this season. His recent poor performance is due to several factors which he has little control over. In the future, regression to the mean in each of these factors and Jake’s better skills should result in the performances O’s fans expected from him after his opening day start.