Roegele: How does changing leagues affect walk rates for batters?

From Jon Roegele at Beyond the Box Score on November 20, 2012:

Glenn DuPaul recently wrote a very interesting article here on Beyond the Boxscore, where he looked at Albert Pujols and his declining walk rate. Reading the article sparked my memory of two other recent players that I recalled had decreased walk rates in National League to American League transitions: Adrian Gonzalez and Prince Fielder. I began to wonder whether switching leagues led to a measurable change in walk rate for the average hitter.

For the purposes of this investigation, I will use the time period from 2002-2012, giving me ten year-over-year change sets for analysis. I chose to look only at players who achieved at least 200 plate appearances in each year. I wanted to make sure that BB% had stabilized in each year of the study, and 200 PA is the most conservative of these two studies regarding metric stabilization. Trying to look at players who changed leagues during a season would add some complexity, in particular when they did not accumulate more than 200 PA on one or both of the teams in that season. To make things simple, I decided to only look at players who changed leagues during the offseason. In other words, players who played all of year N in one league, and all of year N+1 in the other. I also excluded those players who missed the entire N+1 year (due to injury, for example) and changed leagues in year N+2. The sample set of players is thus fairly consistent in the manner in which they changed leagues.

Over the decade in question, walk rates in general have been falling slightly on average in both leagues.

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Originally published: November 20, 2012. Last Updated: November 20, 2012.