Wilin Rosario: Estimating BB and K Using Plate Discipline

In September, teams are allowed to expand their rosters and the Rockies did that in 2011 by calling up Wilin Rosario. Rosario showed a bit of pop, but had some problems making contact. Going into 2012, questions about his ability to not strike out existed. By using a small sample size of a hitter’s swing and contact values, a better estimate of his walk and strikeout rates can be estimated.

The Rockies began the 2012 season with Ramon Hernandez as their #1 catcher and Wilin Rosario was slated as the backup even though Rosario was a highly touted ranked prospect (#49 in 2011, #87 in 2012). The main reason the Rockies didn’t have any faith in Rosario was his plate discipline. In the minors, his BB% ranged between 4.5% and 8.7% and his K% between 19.2%-29.9%. In 57 MLB plate appearances, his BB% was 3.5% and his K% was 35.1%. These values forced people to have reservations about him being able to stick in the majors.

In the 2011 FG+ fantasy preview, Paul Swydan wrote the following on Rosario:

Swinging at every pitch thrown to you is only a good strategy for a hitter if you have enough bat control to hit or foul off nearly every pitch thrown to you (see Guerrero, Vladimir). Wilin Rosario is not this type of hitter, and his acceptable plate discipline in the low minors has steadily worsened as he has moved up the Rockies’ organizational ladder.  ….. Rosario still needs to fine tune his game — particularly his plate discipline — and is unlikely to contribute to your team no matter where he starts the season.

Instead of using BB% and K%, a player’s estimated K% and BB% can be determined by using swing and miss values. To get an idea of this value, I created a formula using (See Appendix) O-Swing%, K-Swing%, O-Contact% and K-Contact% plate discipline values.

By plugging Rosario’s 2011 plate discipline numbers into the spreadsheet, his 2011 plate discipline numbers would be 22% K% and 6% BB%. While the BB% is fairly close to his actual value (4%), the K% is off by 13 percentage points.

With questions surrounding his plate discipline in 2012, he saw is K% end up at 23%. This was within 1% point of what his 2011 estimated K%. With reasonable plate discipline, he was able to put up a decent season (1.8 WAR in 426 PA). Using a second method to calculate a Rosario’s K% and BB% helps to get a better picture of his true talent level.

Rookies, like Rosario, are called up and get a small number of plate appearances. By using a player’s plate discipline numbers, the player’s walk and strikeout rates can be estimated. The estimate can help determine if the player’s talent level is significantly different than their stats suggest.

I wanted a formula to help estimate a player’s K% and BB% using the plate discipline values available at FanGraphs. The formula create wouldn’t be a prediction (as it contains no regression) or stat that stabilizes fast.

I took every player that had over 200 PAs in a season from 2002, when plate discipline numbers are first available at FanGraphs, to 2012. I ran a linear regression against over 3500 seasons and came up with the following two formulas:

BB% = -0.228 x O-Swing% -0.139 x Z-Swing% – 0.030 x O-Contact% -0.257 x Z-Contact% + 0.437
R-Squared = 0.45

K% (K/PA)
K % = 0.248 x O-Swing% -0.345 x Z-Swing% – 0.153 x O-Contact% -0.837 x Z-Contact% + 1.169
R-Squared = 0.79

I have gone ahead and saved people some time and uploaded a spreadsheet to the Google Docs that will automatically do the calculations.

To use the sheet.

1. Download the spreadsheet by using the “Download As” feature under File.
Go to the players page at FanGraphs, minimize minor league data, go to the Standard stat area and copy the all the data going back to 2002.

2. Go to the downloaded spreadsheet and paste the data with the upper left corner being the left yellow box.

3. Go back to the player’s FanGraphs page and copy the (non-Pitch F/X) Plate Discipline values.

4. Go back to the downloaded spreadsheet and paste the data with the upper left corner being the right yellow box.

5. Once the data has been added to the spreadsheet, the player’s real and estimated K% and BB% will be calculated.

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Jeff writes for RotoGraphs, The Hardball Times, Rotowire, Baseball America, and BaseballHQ. He has been nominated for two SABR Analytics Research Award for Contemporary Analysis and won it in 2013 in tandem with Bill Petti. He has won three FSWA Awards including on for his MASH series. In his first season in Tout Wars, he won the H2H league. Follow him on Twitter @jeffwzimmerman.

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