Introduction
Baseball writers – and fans – love to use regression as a catchall for a players performance. Is there a player having a good start to the season or month? He’s probably due for some regression. A terrible slump for a former MVP? Just you wait until that positive regression kicks in. There’s certainly some kernel of truth to this dynamic. We can generally expect that given a large enough sample size, a given number will return to its calculated mean rather than more extreme, one-off results – or in other
words, a regression (or reversion) to the mean. That’s obviously not a comprehensive explanation of the phenomenon and it’s worth reading more on the subject, but it’s enough for our purposes. We’ve now reached just beyond the quarter point of the baseball season so the “small sample” size excuse for player statistics is quickly waning. That obviously doesn’t mean that statistics can’t change, but it does mean that those numbers are becoming more and more representative of actual reality rather than the result of statistical “noise.” Given that fact, I wanted to take a quick dive and see which Arizona players might be most likely to experience a regression – positively or negatively – to their respective means.
Positive Regression
Ketel Marte has been the heart and soul of this team for the better part of the last decade since Paul Goldschmidt was traded back in the 2018 offseason. But it has been an ugly start to his 12th big league season with a .215/.271/.367 slash line, a homer rate that’s been halved year over year, and a strikeout rate that’s bumped up three percentage points. If those numbers held through the end of the year, it would be far and away the worst season of his career. Thankfully, there are some signs underneath those topline numbers that suggest some better results might be in store for the unofficial team captain. He’s the team “leader” in difference between the expected and actual batting average as well as the difference between actual and expected slugging. Similarly, he’s the owner of a miserable .236 batting average on balls in play (BABIP) which implies that he’s probably getting a little unlucky and which hopefully will settle out over the course of the season. There are a few indicators that show he’s starting to slow down a bit in his age-32 season with his chase, walk, and barrel rates all deteriorating.
Negative Regression
Eduardo Rodriguez and Ildemaro Vargas have been some of the best success stories of this young season for the D-Backs. After two years of mediocrity from Rodriguez and an entire career’s worth of replacement-level performance for Vargas, they’ve been among the team’s leaders and have been large reasons why the team is where it is at this point. Unfortunately, there are red flags all over indicating that they’re due for some backsliding. Both of them are outperforming their expected statistics by a decent margin – 25 points of batting average for Vargas and nearly two whole runs for Rodriguez. For Vargas, he’s also seeing some poor numbers under the hood with significantly below-average bat speed, hard hit rate, and barrel rate translating to an absurd .336 BABIP. It’s obviously possible that all of the changes in results (doubling his home run rate and walk rate, decreasing his groundball rate, etc) can be attributed to changes in mechanics or approach, but I’m not astute enough to know if that’s the case. Meanwhile, Rodriguez seems to be posting his results with some serious smoke and mirrors. Amazingly, his strikeout rate, whiff rate, and hard hit rate have all gotten worse from last season when Rodriguez posted a 5.02 ERA and .286 BAA. This year, those numbers stand at 2.25 and .202 respectively with an absurd .227 BABIP and 4.19 FIP so I’ll be shocked if his excellent start to the season continues to this level rather than a slide back to his career norms.
