23 December 2015

Don't Worry About Chris Tillman's K-BB% Decline

Chris Tillman is the icing on the cake of the Erik Bedard-Adam Jones swap of 2008. If the team had gotten only Jones, the trade would’ve been a win. He has become the face of the franchise during the Orioles’ return from exile in baseball’s hinterlands. But Tillman has provided good value also. Although he struggled from 2009-2011, he pitched well from 2012-2014, breaking out alongside Jones, Manny Machado, Chris Davis, and others.

2015 was his worst full season yet. Hidden within the high ERA was the fact that the season concluded a three-year stretch where his K-BB% dropped each year, even when accounting for changes in league K and BB rates.

Here are his K-BB rates relative to the league, where 100 is average and higher is better:

  • 2013: 114
  • 2014: 78
  • 2015: 61

This drop alarmed me. Strikeout and walk rates are among the most relevant predictors of a pitcher’s future success (in terms of run prevention). Accordingly, I wanted to understand whether this drop portended doom for the 6’5”, 210-pound righthander.

To check, I gathered all pitchers since 1955-2010 who met the following criteria:

  • Starting at age 24, 25, or 26, experienced two consecutive year-over-year declines, relative to the league each year, in K-BB rate
  • Did not miss a season in this three-year window.
  • Pitched at least 150 innings each year.

120 pitchers met these criteria, 1.3% of pitchers in the 8,944-person sample. The names range from all-time greats to scrubs you’ve never heard of. For all 120 pitchers, I then looked up the following information:

  • Their average number of innings pitched each season for the three seasons following the decline
  • Their ERA- in these seasons.

In this study, the first metric is the best one to indicate how much value the Orioles might get out of Tillman in the next three years. A high innings total means a pitcher is both healthy and good. A low innings total would indicate the opposite, with the possibility that a starter got moved to the bullpen. The length of time is relevant because the Orioles control Tillman for the next three years as they go through salary arbitration.

We can look at this set of results in a variety of ways.

Largest and Smallest Three-Year Declines

The largest three-year K-BB decline happened to Mario Soto from 1982-1984. In those years his K-BB rate declined a whopping 248%. In the three years following his decline, he was demonstrably worse than league average, although not a bad pitcher, pitching 130 IP/year with an ERA- of 112.

The smallest decline happened to two pitchers: Mike Pelfrey from 2008-2010 and Jim Bouton from 1963-1965. Each declined by 9% relative to the league. Pelfrey, who recently signed with the Tigers, averaged 121 IP/year in the three years following his decline, even though his ERA- was a respectable 106. The reason for his low innings total was that Tommy John surgery that cost him most of the 2012 season. His K-BB total actually declined one more year in 2011.

Bouton averaged just 69 IP/year in the three years following his decline with an ERA- of 119. Okay, but not good. The first year after his decline was his last year as a starter; the next two years he was a reliever/spot-starter. (It must be said that in the fourth year after this decline, he signed with the expansion Seattle Pilots and wrote a famous book about the experience.)

Best and Worst Post-Decline Phases

When I say "best" here, I mean "highest average IP/year for the three seasons following the decline". As I said above, a high IP total is an indication the pitcher is performing well. The reason he's doing so is not important for this study.

The best post-decline phase has to be that of Jim Palmer. Yes, Tillman can take solace in the fact that the man who calls half his games also experienced a significant decline in K-BB rate. From 1972-1974 Palmer’s K-BB rate fell by an ugly 107%, finishing at a low of 1.9% in 1974. That wouldn’t play today, but in the mid-70’s that was okay, and Palmer was still getting outs. He continued to do so for the next three years, averaging a whopping 319 IP/year with an excellent ERA- of 70.

Another famous name comes in second place on this list. Steve Carlton’s relative K-BB rate declined 78% from 1969-1971. That didn’t hurt his career any, as over the next three years he averaged 310 IP/year  with an ERA- of 81.

The worst post-decline "career" is undoubtedly that of Noah Lowry, who declined 135% from 2005-2007 and didn't pitch at all after that. The thing is, he was a very good pitcher in 2007, putting up an ERA- of 88. He just suffered a string of injuries that forced him out of the game. Down in the cellar with him is Ricky Romero, whose K-BB rate declined 88% from 2010-2012 and who's thrown just 7.1 major league innings since.

Does K-BB% Decline Explain Variation in Post-Decline IP?
In addition to Palmer and Carlton, several other Hall of Famers show up on this list. Nolan Ryan, Don Drysdale, Dennis Eckersley, Sandy Koufax, Bert Blyleven, and Tom Glavine are all there. Other names you’d recognize are Orel Hersheiser, Dave Stieb, Dwight Gooden, Jon Lester, Andy Pettitte, James Shields, Kevin Brown, Josh Beckett, Carlos Zambrano, Tommy John, Tim Hudson, Cole Hamels, and Max Scherzer.

The presence of such names shows that you can be a good pitcher, even a great one, while experiencing a declining core skillset at a young age. A scatterplot shows conclusively that the magnitude of a pitcher’s K-BB rate decline explains none of the variance in how many innings he will average during the following three years:

Soto, way out by himself on the right side of the graph, probably skews the results, as does the man to his left (Mike Norris with a 203% decline from 1980-1982). If we examine pitchers whose K-BB rate declined between 43 and 63%, a range that places Tillman in the middle, do we get a different picture?

Nope. The relationship is just as non-existent even when looking at Tillman’s direct comparables.
These graphs show that K-BB rate declines happen to Hall-of-Famers, middle-tier innings-eaters, and scrubs alike. If you need further proof, here’s a histogram of post-decline ERA-:
50% of these pitchers have an ERA- of 103 or lower, with the average being 111. The Orioles would take this kind of performance from Tillman; it would certainly beat his 2015 ERA- of 121.

There is probably some survivor bias going on here. If you’re good enough to pitch 150 innings/year for three years starting at age 24, 25, or 26, you’re good enough to compensate for a K-BB rate decline. That, or perhaps strikeouts were never a big part of your game to begin with. Or you pitch to contact enough to limit the damage from walks. Or you're young enough that some team will give you a chance.

Tillman has shown some evidence of adjusting; as his K-BB rate has declined, he’s become more of a ground-ball pitcher in an effort to limit the damage:

This adjustment doesn’t mean that Tillman will be okay, but that indicators of his decline will have to come from somewhere else. The fact that his K-BB rate has declined means nothing for what the Orioles should expect out of him through 2018. Plenty of major-league pitchers have experienced what Tillman is going through and have been just fine. 

You can view the full list of K-BB decliners here.

All data for this post courtesy of FanGraphs.

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