Quickie: Jay Bruce is off to a very wonkish start. April 14, 2017

Posted by tomflesher in Baseball.
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In an earlier post, I noted that Jay Bruce would have been expected to accumulate about .0838 walks per plate appearance (whether that’s estimated using a linear time trend or a quadratic one). It’s interesting to note that Bruce hasn’t merely hit more home runs and taken more walks so far, he’s also struck out significantly less.

Using R, I simulated 1,000,000 46-plate-appearance streaks, using a binomial trial system (simulating only “struck out” or “didn’t strike out” as outcomes). Choosing what rate to use for Bruce’s expected rate of strikeouts per plate appearance (K/PA) was a challenge. Last season, he had a historically low K/PA rate of .2139 (one strikeout ever 4 2/3 plate appearances) – bested only by his 2009 season (.1938). His rate with just the Reds was .2065 and his rate with just the Mets was .2299. Estimating a time trend was difficult because Bruce has fluctuated quite a bit, yielding .2429 for a linear model and .207 for a quadratic model. His total strikeouts through the end of 2016 (1239) divided by his total plate appearances (5189) give a rate of .2388. A simple average of his yearly K/PA numbers gives .237.

Since the condition I’m testing is whether Bruce has improved markedly, I’ll take the logical measure that contains the most likely bias – that is, I’m going to take a low one. I went with his .2139 mark from last year, since it combines his likely unusual rate with the Reds with a more reasonable, but still good, rate with the Mets. (I know, it’s wild that he was more disciplined than he’d been most of his career during the time he was being crucified in the media, but so it was.)

Those 1,000,000 streaks of Bruce’s 46 plate appearances yielded a distribution clustered around 9 and 10 strikeouts. The mean was 9.841 and the median was 10. About 28% of the distribution was contained in the 9- and 10-strikeout blocks. As for his actual number, Bruce has only struck out 4 times. The likelihood he’d have 4 or fewer strikeouts at this point is only about .02, meaning it happened about 2% of the time. That figure would be even smaller if we assumed a higher rate of strikeouts.

When strikeouts decrease at the same time walks increase, there are a few possibilities. It could be that the small sample size is emphasizing Bruce facing worse pitchers than usual, or umpires who are more batter-friendly than usual. Ruling out those possibilities would simply require examining how the rest of the Mets are faring compared with previous years. It’s also possible Bruce is simply on a hot streak and will cool down to his expected levels soon. Nevertheless, he’s been batting smart this year, including finally bunting through a shift last night in a crucial play to come back from an early deficit.