A Look at Marcus Semien and BB%+K% Change

Since I did not own him on any of my fantasy teams in 2019, I was not aware of the incredible season Marcus Semien was having until it was almost over. His WAR of 7.6 was a career high and good for 5th best in the MLB. Almost two weeks ago Grey, the lead writer of Razzball.com, posted the recap of the top 20 SS of 2019. In standard 5×5 leagues, the Razzball player rater had Semien as the sixth best shortstop for fantasy this season. In past years, it might not mean much, but this year there were a lot of productive players at the position so ranking sixth was no small feat. In his recap of Semien’s season, Grey provided an interesting tidbit I will quote below: “His Ks dropped from 18.6% to 13.7% and walks went up from 8.7% to 11.6%. Ready for some wonky math? Great! Almost 5% down on Ks and 3% up on walks, so that’s 8% total difference. I wonder what the highest percentage difference is for a player in 2019 who had at least 2500 previous major league at-bats. I have to think Semien’s close to the best, or edging near.” Normally when writers pose these type of questions, I think about answering them, but never do. However, that day I decided to do just that on my lunch break. This article details my journey to answering this question. Did I successfully complete it? Was Grey right? Keep reading to find out. How about that for a cliff hanger?


Besides actually taking words and forming coherent sentences that make sense to not just myself, the hardest part of researching and writing for me is coming up with a worthwhile question to answer. With the question already asked and criteria set, all I had to do was retrieve the proper data and transform it appropriately. I was looking for the total positive change in BB%+K% from 2019 to 2018. Therefore, I needed to find the positive change in BB% and positive change in K% before summing them together. Fortunately, I was able to find this data at Fangraphs.com.

Once there, I exported three distinct data sets. The first was all 2019 batters with a minimum of 0 PA. The second was all 2018 batters with a minimum of 0 PA. The third was all batters from 2001 to 2018 with with at least 2500 career PA (Note: Upon editing this piece, I realize now that the original question called for a minimum of 2500 AB and not 2500 PA. I am going to leave it as is because the analysis is sound). I picked 2001 as an arbitrary start point as I wanted to make sure I included all current batters. That said, 2001 was probably being overly cautious. I put all three of these individual, extracted .csv files into one excel workbook. My master sheet is the list of 2019 batters with the other two sheets, referred to from now on as 2018 and 2500 respectively, serving as a reference for filtering down the 2019 data set.

I used the functions index and match (equivalent of a vlookup) to determine if a 2019 batter had previously had 2500 career plate appearances by seeing if they appeared in sheet 2500. This filtered down my 2019 sheet from 1410 batters to 126. For the remaining batters, I again used the index match functions to search for their their 2018 BB% and 2018 K% from the 2018 sheet. To indicate a positive change in BB%, the 2019 metric has to be higher than the 2018 metric. I created a column where I subtracted 2018 BB% from 2019 BB%. To indicate a positive change in K%, 2019 metric has to be lower than the 2018 metric. I created a column where I subtracted 2019 K% from 2018 K%. Summing these two new columns gave me the total change in BB% and K% I was looking for.


As it turns out, Grey’s hunch was indeed correct. Of all batters that received a qualified number of PA in 2019 and had already had at least 2500 PAs prior to this season, Semien has the greatest positive change in total BB% and K% with a total figure of 7.8%. This does seem like an incredible achievement, without context, and it makes sense that a player would improve in both BB% and K% should have a great season. However, to add some context to this achievement of Semien, it would be worth looking into how he stacks up against batters from previous seasons as well as looking to see if batters who accomplished this feat continued to show positive gains in the following year (I may do this). Total results of the final data set can be found at by clicking this link.

If we lower the PA threshold for 2019 to 0, but keep the other criteria, Semien falls to third behind Howie Kendrick (9.1%) and Mark Trumbo (8.0%) respectively. Like Semien, Kendrick had himself a very good year, but just in a part time capacity. Yet, in his 14th MLB season, he put up the third most WAR of his career (2.9) with by far his highest wRC+ (146) in only 370 PA! He is probably worth writing about on his own, though I am sure plenty of others will after his post season heroics (Note:I wrote most of this article prior to the Nationals making it to the World Series, thanks in part to Kendrick, so my statement was correct). As for Trumbo, he only had 31 PA this year due to a leg issue that sidelined him for most of the year. He is not known for his plate discipline and had a down 2018 so we can chalk this up to some regression and a small sample size.

Thanks for reading and have a great day!

Original post on IVthoughts.com. All data taken from Fangraphs.com.

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