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The Saber's Edge: Second Half Skill Changes

Jeff Zimmerman

Zimmerman writes analytics-focused baseball and football articles for RotoWire. He also handles scouting and reporting for and contributes to, and Jeff is also a two time FSWA award winner, including the 2016 Football Writer of the Year award.

Last week, I looked at ways to find which hitters may be having a second half renaissance. This week, it is the pitchers' turn and the search is much easier. I went to the root of successful pitching...strikeouts are good and walks are bad. By comparing first and second half strikeout and walk rates, I found pitchers that are struggling or improving as this season has gone on.

In my first article at RotoWire, I ranked pitchers using early-season K% and BB% values. The reason behind just looking at K% and BB% is that they stabilize quickly and give us an idea of a pitcher's true ability. Additionally, subtracting BB% from K% creates a great estimator of a pitcher's talent.

By using these values, I compared the change in each pitcher's K%-BB% value from the season's first half to the second half (minimum 10 IP in both the 1H and the 2H). Also, I removed intentional walks from the equations. I don't think a pitcher should be punished because of a decision that is out of his hands.

Warning/Caution/Don't jump to conclusions reminder: Remember that pitchers who move from the bullpen to the starting rotation or vice versa, will see their numbers change. A pitcher will almost always have better numbers as a reliever than as a starter.

Let's go ahead and look at a few of the most extreme cases.


Shawn Tolleson (+18%) - The Rangers' 26-year-old right-handed reliever is having quite a second half. The main reason for the improvement is a ~11% jump in his strikeout rate. A couple of related changes have led to the turnaround from a 3.47 ERA and 1.20 WHIP, to a 1.17 ERA and 0.98 WHIP. First, he has seen his fastball go from averaging 92.2 mph in March to 93.8 in August. A ~1.5 mph increase in velocity will undoubtedly help strikeout numbers. Additionally, he went from throwing his 'faster' fastball just 58% of the time, to using it 87% of the time.

Besides the strikeouts, his walks have dropped. It may seem higher at other places on the vast internet, but intentional walks are inflating his walk rate. Of the five walks he has given up in the second half, three have been intentional. By removing the intentional walks, his true talent becomes more apparent.

With Neftali Feliz's injury history, I could see Tolleson fall into the Rangers' closer role at some point next season.

Nick Martinez (+12%) Another Ranger, but this one is in the rotation. The 23-year-old was basically forced onto the major league staff with all of the injuries on the parent club. To begin the season, Baseball America had him ranked as the 12th best prospect in the Rangers' farm system. From BA's 2014 Prospect Handbook:

"His solid three-pitch mix and knack for generating groundballs could make him a No. 4 starter with an outside chance to reach Texas in 2014...."

As expected, the rookie struggled in the first half with a 5.98 FIP. He has improved all of his inputs into FIP in the second half.

Stat: 1H, 2H
K%: 9.5%, 20.1%
BB%: 10.2%, 7.9%
HR/9: 1.6, 0.8

Doubling a pitcher's K% and halving their HR/9 will make them a better option. The biggest improvement has been a change in his confidence, as seen from his willingness to throw more pitches in the strike zone. He threw only 44% of his pitches in the strike zone in March thru June. In July and August, his Zone% increased to 50%. He is a pitcher I will watch in September to see if he can sustain the gains.

Chris Tillman (+12%) - Tillman has really turned it on the second half as seen by his K/BB going from 1.5 to 5.3. He hasn't gone to the Phil Hughes level of not walking anyone, but he is around the Bartolo Colon level. He really hasn't changed much otherwise over the time frame. His Zone% has increased from 50% to 53%. He cut his changeup use in half. Both are not enough to explain the overall change.

I would not be surprised if he was pitching through some pain in the middle part of the season. He missed a start for a groin pull late in May which may have taken a while to recover from.

Month: K%-BB%
Apr: 13%
May: 4%
June: -2%
July: 9%
Aug: 18%

The mid-season struggles seem behind him and he is currently pitching like his hair is on fire.


David Robertson (-27%) - Man, he came down to earth after an amazing first half. While he is still putting up good numbers, a 20% drop in K% is insane. Additionally, his BB% nearly doubled (7% to 13%). He went from elite level to OK-ish. His K%-BB% in the second half (11.6%) is actually below the overall league average value of 12.5%.

He has lost ~1 mph off his cutter over the course of the season. Even though his velocity is down, it doesn't explain this much production loss. The truth is that Robertson was due for K% some regression considering his swinging strike rate (SwStr%). Historically, K% = 2 * SwStr%.With that knowledge, here are his 1H and 2H values:

1H: 12.7% SwStr%, 45.0% K%
2H: 11.9% SwStr%, 24.5% K%

His SwStr% supports a ~24% K% which is how he's performed in the second half. His first-half K% was pretty much impossible to maintain with his SwStr%.

Greg Holland (-16%) and Aroldis Chapman (-14%): Like Robertson, they both started the season off with insane K% (Chapman's is still pretty much there), but their K% has leveled off closer to what their SwStr% will support.

Greg Holland
Half: SwStr%, K%
1H: 15%, 39%
2H: 13%, 27%

Aroldis Chapman
Half: SwStr%, K%
1H: 20%, 54%
2H: 17%, 48%

Jason Hammel (-15%) - The 31-year-old has struggled since being traded to Oakland (only one of his 2H starts was with the Cubs). Both his strikeouts and walks are headed in the wrong direction, which can be seen by his K/BB going from 4.1 to 1.6.

His fastball velocities are steady, with a small drop in his changeup and slider speeds. His pitch usage is pretty much the same as the first half.The decline comes down to AL hitters not chasing pitches out of the zone and when he does have to throw in the zone, hitters are pouncing on the meatballs.

Half: Zone%, Swing%, Contact%
1H: 43%, 46%, 76%
2H: 47%, 43%, 84%

The AL hitters are making him throw more strikes and then crushing them. I am not sure how he is going to right the ship.