Where have Kevin Kiermaier's walks gone?

Kevin Kiermaier is having a fantastic year as a baseball player, and he's one of the most valuable pieces the Rays have. That being said, his offense really hasn't been very good. While he was above-average with the bat in 2014 over 364 plate appearances, he's been below average this season in 376 plate appearances.

Let's start out by stating a few things that are not the problem:

• Kevin Kiermaier is not facing more lefties in 2015. Both this year and last, he's faced a left-handed pitcher in 22% of his plate appearances.
• Kevin Kiermaier is not having significantly worse balls-in-play luck. Last year, his BABIP was .306, while this year it's .294. That's a difference, and it's powering his lower batting average, but with tatistics as noisy as average and BABIP, the difference is negligible and really not worth looking into.
• Kevin Kiermaier is not striking out more often. Last year his strikeout rate was 19.5%, while this year it's 19.1%.

Instead, two things have happened. His isolated power has fallen from .187 to .151, which is a rate more in line with what he produced in the minor leagues, and his walk rate has fallen from 6.3% to 3.7% Unlike with isolated power, KK has never had much trouble taking a walk in the minor leagues, so I think that the walk rate is worth digging into a bit more.

To get the big picture, I've reproduced the statistics from the PITCHf/x plate discipline section on FanGraphs:

From this vantage point, it seems pretty clear. Kiermaier is swinging more overall, on pitches both inside and outside of the strike zone, and he's making contact on more of those pitches outside of the zone. That lowers his walk rate in two ways, because it both turns balls into strikes (the higher O-Swing%) and it ends at bats earlier before they're able to progress to four balls (the higher overall swing % and the higher contact %).

With this in mind, let's now take a look at Jeff Zimmerman's excellent swinging/taking tool on Baseball Heat Maps, because it allows us to compare Kiermaier in 2015 to himself in 2014. A "hotter" color means that he's now swinging at a pitch in that location more often while a "cooler" color means that he's taking a pitch in that location more often. All heat maps are from the catcher's perspective.

For simplicity, and because it's driving the majority of KK's change in walk rate, I'm just going to show the data vs. right-handed pitching.

First off, here is Kiermaier against fastballs (FA, FF, FT, SI):

While he's actually taking a few more pitches straight down the middle, it looks like Kiermaier is being more aggressive on the edges of the strike zone, particularly up and on the outside. Some of those pitches near the edge are strikes (which balances his takes of fastballs down the middle in the overall numbers), but many are balls (which nudges his O-Swing% higher).

Now here are are the offspeed pitches (CH and FS):

This looks like an objective improvement in approach. The only change is that Kiermaier is more apt to take changeups on and off the extreme outside-bottom corner. Most of those are balls, and the ones that are strikes are pitchers' pitches, moving down and away from him in a perfect spot (so probably not something he can hit with authority anyway).

Finally, here are the breaking balls (SL, CU, KC):

Kiermaier is taking more of the backdoor breaking balls he sees, but he's going after low breaking balls, both at the bottom of the strike zone and below it. Unlike Steven Souza Jr., he's actually making contact with many of these pitches, which to my mind indicates that there's some intentionality to it -- he's protecting the plate rather than simply being fooled, perhaps. Still, this is a situation where I'd rather see Kiermaier take that low breaking ball (which, like the outside-corner changeup, is not something he'll often be able to hit well).

Conclusion

Top-down statistics like the FanGraphs strike zone numbers are useful, but as often becomes clear when one looks more granularly, they can also be misleading. Kiermaier is swinging slightly more often overall, but he's arriving at those numbers by swinging more often in some locations and less often in others. While his approach against changeups looks solid, against both fastballs and breaking balls, he's shifted his swing pattern away from some pitches more likely to be called strikes and towards some pitches more likely to be called balls, which will obviously result in a lower walk rate.

The areas where Kiermaier is being more aggressive this year are:

1. Fastballs at the top of the zone.
2. Fastballs on the outside.
3. Breaking balls at the bottom of the zone.