Last week, I wrote an article about pitch location and swing tendencies of this year compared to last year to investigate why the Royals’ strikeout rate is up so much this year. I mentioned following that up with looking into pitch types. Due to circumstances, I can’t give a full follow-up of that this week, but I did find a little nugget of interesting information.
I downloaded FanGraphs’ PITCHf/x data for each team’s 2015 and 2016 seasons to look at pitch types. The Royals have definitely seen some change.
From 2015 to 2016, the Royals have seen the second-highest increase in breaking ball usage (CU% + SL% + KC%) against them. They were in the middle of the pack last year in this rate, and they rank first this year. Given their propensity to swing at things outside the zone, this would be an easy factor to pinpoint as one of the reasons for the increased strikeout rate.
They’ve concurrently seen almost a four percentage point decrease in FA% (four-seam fastball) in the same timeframe. This is certainly a recipe for increased strikeouts. I can’t really say how much. I honestly don’t know how much a one percentage point change in breaking balls seen affects whiff rate / strikeout rate.
With my small-sample 2015 and 2016 data, this breaking ball percentage was better correlated with strikeout rate than whiff rate, but neither correlation was particularly strong. In the case of strikeouts, a one percentage point increase in breaking ball usage resulted in about a quarter of a percentage point increase in strikeout rate (~0.24%). The trend was neither significant nor strong, so don’t read too much into that. More data are needed to make any more solid conclusions from this.
Basically, I think the increased breaking balls is part of the problem, but it definitely doesn’t explain the whole thing. Not even close.