Bruce Chen has had a pretty weird career. He was a top prospect with the Braves, got to the Majors at 21, and had his first excellent season at age-23. And then he fell apart and became the definition of a replacement level pitcher. From 2000 to 2003, he played for the Braves, Phillies, Mets, Expos, Reds, Astros, and Red Sox. He changed teams at least once in each of those four years, and was officially a journeyman by the time he was 26.
A decade later, he just re-signed with the Royals for another $4.25 million in guaranteed money, with an option that could actually keep him around through the end of his age-37 season. This wasn’t a particularly easy outcome to see coming, given how mediocre he was for most of his career, but his late career revival is almost something of a reminder about just how much of a pitcher’s performance is driven by home run rates.
From 2002 — the first year we have batted ball data here on FanGraphs — through 2009, Chen was as HR prone as any pitcher in baseball. His home run rates were literally off the charts. And I do mean literally.
His 2.55 HR/9 allowed in 2006 was the second highest home run rate ever allowed in a season where a pitcher threw 90+ innings. Chen was basically throwing home run derby for the better part of a decade.
And then he basically just stopped giving up so many home runs. After posting a 14.8% HR/FB rate from 2002-2009, he’s at 8.9% since, spanning over 600 innings in the last four years. While his extreme fly ball tendencies ensure that he still gives up his fair share of home runs, he’s no longer leading the league in HR rate, and the three lowest HR/9 seasons he’s had in his career have come in the last four years. In fact, 2013 was the first time in his career where his HR/9 was under 1.0.
Interestingly, there hasn’t really been any significant improvement in the other parts of Chen’s game. His K/BB ratio has actually gotten slightly worse, relative to league average, than it was earlier in his career.
He’s still basically the same guy he’s always been, a average walk/low strikeout/fly ball pitcher. Only recently, he’s been a decent version of that pitcher type, because he’s giving up one home run per game instead of two. And by chopping his home run rate from 1.8 HR/9 to 1.2 HR/9, he’s gone from replacement level scrub to a reasonably solid innings eater. From 2002-2009, Chen was worth about +0.5 WAR per 180 innings, while from 2010-2013, he’s been worth about +2.0 WAR per 180 innings. The drop in home run rate basically added +1.5 WAR per season to his overall value, even with no other real dramatic changes to his overall profile.
And this is one of the primary things that makes projecting the results of any pitcher in any given season so difficult. Home run rate has a massive influence on a pitcher’s overall performance, but is also extremely volatile on a seasonal basis. The year to year correlation for home run rate is around half of what it is for things like walk rate and strikeout rate, and while pitchers can and certainly do improve their BB and K ratios, the magnitude of the changes are hardly ever as large as what we see from the variance in home run rates.
For almost an entire decade, Chen was a batting practice pitcher who gave up home runs at absurd rates. And then, with no real explanation, in the midst of the decline phase of his career, he stopped giving up home runs and became a decent pitcher. We can come up with post hoc explanations as to how he’s done it, but I’m pretty sure it’d be impossible to find anyone who saw this coming ahead of time. And that’s a lot of what we try and do in baseball forecasting; identify changes before they happen. But home run rates thwart us more often than we’d like to admit, because even if we nail everything else about a projection, a big increase or decrease in home run rate can overwhelm every other part of a pitcher’s performance.
Just look at 2013 Matt Cain, who saw his home run rate spike for the first time in his career, and went from an All-Star to a problem in the process. Or Dan Haren, who is still running elite K/BB ratios but is now a fifth starter on a one year contract because he’s spent the last two years giving up home runs left and right. Home run rates can dramatically alter the perception of a pitcher’s quality, even though on a year to year basis, we don’t really know how many dingers a pitcher is going to allow.
This simple fact is one of the reasons why a run estimator that is solely based on BB/K data can actually hold its own against more complicated metrics. We can do all the adjustments we want, and account for every variable we can think of, but if the home run rate swings one way or another, it overwhelms nearly everything else. Home run rate isn’t the only determining factor of a pitcher’s success, but it’s a big factor, and it’s a very difficult one to forecast.
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