Bruce Chen and the Power of the Home Run

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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Dave is a co-founder of and contributes to the Wall Street Journal.

32 Responses to “Bruce Chen and the Power of the Home Run”

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  1. E says:

    Are you suggesting Chen was just unlucky for 7 years?

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    • JayT says:

      No, I think he’s saying that homerun rate can turn a good pitcher into a bad one and vice versa, but we also don’t have any reliable way to know what a single-season HR rate will be.

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  2. CleverName says:

    How much of the drop should we attribute to Kauffman stadium?

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    • TangoAlphaLima says:

      You would think quite a bit. The last 4 seasons Dave is referring to are Chen’s only 4 full seasons with the Royals (he pitched 62 innings with them in 2009). And Chen’s previous two home ballparks were in Arlington and Baltimore, not exactly where a fly ball pitcher would want to pitch half of his games.

      But going through Chen’s HR/FB home and away splits over the years, while it is clear being in Kauffman has helped, the change in home ballpark hasn’t done it alone. Before he was with the Royals, Chen had an awful HR/FB ratio no matter where he went. It’s much better now, but it’s only slightly better at Kauffman than on the road. (With the caveat that other than U.S. Cellular Field, the road games Chen plays in the division are all in pitchers parks as well, so the division he plays in may also help keep the away HR/FB ratio down as well.)

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  3. jpg says:

    A lot has to do with sequencing. Johan Santana immediately comes to mind as a guy who always gave up a lot of homeruns but always out performed his FIP because the homeruns he allowed were rarely of the multi run variety.

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  4. schlomsd says:

    With pitch/fx can’t we just look to see if the location of his pitches have changed?

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  5. Mike Green says:

    There were nine qualified starting pitchers with a HR/FB% of 13 or higher over the 2010-2012 period. In 2013, two were out of MLB (Blackburn and Rodrigro Lopez), two were worse than ever in this regard (Jason Marquis and Joe Blanton) and five were better and had relatively typical HR/FB rates (Hunter, Leake, Volquez, Burnett and Correia). There are some notable overall improvement candidates there. Henderson Alvarez missed the cut for qualification, but the drastic reduction in his HR/FB from 16.5 to 2.6 between 2011-12 and 2013 is what fuelled his improvement.

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  6. GilaMonster says:

    I don’t really get this signing. Why not go out and sign a Chris Capuano type for a tad more money? Or sign a Josh Johnson for a bit more? Or even a Shaun Marcum?

    Instead you sign a bum like Bruce Chen

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    • TangoAlphaLima says:

      Because Dayton Moore.

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    • KCDaveInLA says:

      Because those three have had health issues and Bruce Chen has not?

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      • GilaMonster says:

        A 2.0 WAR pitcher or a pitcher that could hit 2.0 WAR in half a season with upside?

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        • TangoAlphaLima says:

          My thoughts as well, especially since Chen is mostly an unnecessary piece. He’s not going to be in the starting rotation unless there’s an injury, and he’s not an important part of the bullpen. Should have gone for someone with upside, but Dayton Moore is too loyal.

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        • Ruki Motomiya says:

          Don’t forget the 2nd one will sometimes get you, for example, 0.5 WAR Josh Johnson of last year. Not to mention injuries impact performance. I’d prefer Josh Johnson too, but I can understand trading some upside for stability.

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  7. tz says:

    Bruce Chen is now has 10 and 5 rights and can veto a potential trade.

    Let that one sink in.

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    • TangoAlphaLima says:

      Thankfully this isn’t a factor, because NO ONE ELSE WOULD WANT HIM ANYWAY.

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      • tz says:

        lol and also because DAYTON MOORE WOULDNT TRADE HIM ANYWAY.

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      • Ruki Motomiya says:

        I don’t think most teams would mind a pitcher of 1.8 WAR, 2.0 WAR and 1.4 WAR the last 3 years at 4.5 million.

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        • TangoAlphaLima says:

          If the Royals were planning to use Chen as a #5 starter, it wouldn’t be a problem. But they aren’t. Chen’s gonna be a swing man type. He’ll be in the end of the bullpen, used for long relief or possibly as a starter if injuries occur. This will seriously limit his WAR, and thus his value. A budget-conscious team like the Royals should not be spending 4.5 million on that type of player.

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        • chuckb says:

          $4.5 M is worth about .7 WAR. Even if Chen’s a swingman who throws 120-150 innings, he’ll still probably get to .7 WAR if he continues his HR/FB rate.

          There’s hardly an example of a bad 1 year, $4.5 M contract.

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        • Catoblepas says:

          exactly. you can throw $4.5mil/1 year at basically anything and it really can’t be that bad. the bar is very low, and i’m a fan of this move.

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  8. WATTs? says:

    1.7WAR in 130 innings in 2000. FIP of 4.28. Excellent? I think not.

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  9. jim fetterolf says:

    As I recall, Chen came up as a power pitcher, got hurt, then reinvented himself with the Royals into a crafty lefty with six pitches and three arm angles who keeps hitters off balance and not making square contact. Chen’s best days are with an ump with a fat strike zone and a solid north wind. Give him a tight zone and a south wind and it can be batting practice still.

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  10. John C says:

    I don’t know if you can really consider Chen in the “decline phase” just because of his age. Crafty lefthanders like him can have their best years in their late 30s and it’s not unusual.

    My guess is that as long as he can throw 80, change speeds, and pitch in Kauffman Stadium or some other park that’s good for a fly-ball pitcher, he’ll continue to be successful.

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  11. blue says:

    I think one of the lessons is to never really give up on talent.

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