## Back-to-Back: When Starters Face the Same Team Twice in a Row

Chris Sale had to follow up his complete game one-hitter against the Angels in May by facing the Angels again. - James Guillory-US PRESSWIRE

The MLB schedule is such that at times, two teams will face each other in two series close enough together that a starter will make consecutive starts against the same opponent. Does the opposing offense gain an advantage seeing a pitcher twice in short succession?

Recently I investigated how pitchers change the types of pitches that they are throwing as they face hitters multiple times within the same game. I also noted how their level of success drops on each occasion through the order. One would deduce that this relative advantage is gained by the hitter at least partly to do with their becoming more accustomed to a pitcher, his rhythm, velocity, movement and sequencing.

To extend this idea a little further, I wondered what happened when an offense got to face the same starter twice within a week. Would hitters maintain this learned advantage? Would starters resort to a different game plan when forced to face the same team again right away?

For the study, I decided to look at all occasions in the last three seasons where a starting pitcher faced the same opponent in consecutive starts. The results are shown below:

Time Faced wOBA Against HR% K% uBB% SwStr%
1 .328 2.65% 18.82% 7.45% 7.84%
2 .328 2.30% 17.48% 7.37% 7.77%

Starters Facing Same Team Twice in a Row, 2010

Time Faced wOBA Against HR% K% uBB% SwStr%
1 .323 2.70% 17.57% 7.25% 7.73%
2 .320 2.96% 16.42% 6.13% 7.91%

Starters Facing Same Team Twice in a Row, 2011

Time Faced wOBA Against HR% K% uBB% SwStr%
1 .310 2.79% 18.73% 6.97% 7.86%
2 .318 2.72% 18.70% 7.62% 8.08%

Starters Facing Same Team Twice in a Row, 2012

Overall there does not look to be any sufficient evidence that the opposing offense gains an advantage in these situations. The only consistent trend is that K% drops in the second meeting compared to the first, although there is not a corresponding decline in SwStr%.

A related question would then be: are pitchers making adjustments to their game plan when facing the team the second time? Are they throwing different types of pitches at different times in the game?

At the most basic level, there is some evidence that pitchers do in fact throw less fastballs the second time around.

Year Fastball% Delta
2010 -1.0%
2011 -0.5%
2012 -1.0%

Starters Facing Same Team Twice in a Row, Fastball Usage Delta 2nd-1st Start

These differences are quite small of course, on the order of one pitch per start. In breaking down each start by times through the order, the slightly lower fastball percentages are spread quite equally throughout the start, so there is no evidence of a more concerted effort to throw less fastballs the first time through the order in the second start, for example.

Of course just looking at pitch type frequencies does not identify potential changes in pitch sequencing, which I suspect may be more likely to yield differences when starting pitchers find themselves in these situations.

There is nothing earth shattering learned here, but as far as applying these results, at least if you play fantasy baseball, you can relax when one of your starters faces the same team in back-to-back starts, as we know now on average they will fare just about as well, save perhaps a slight hit on the strikeout rate.

Credit and thanks to Baseball Heat Maps for PITCHf/x data upon which this analysis was based.

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