Effect of Days Rest on Reliever's Performance

Much is said about how many days rest a reliever is performing on. A closer could come into a game after pitching the day before, blow the save, and fans and the media will blame it on him pitching the previous game. Today we will see if there is any evidence to support that theory. The data we look at will be career ERAs of active pitchers with at least 100 career saves on given days of rest. There are thirty pitchers who fit this criteria. For pitchers such as John Smoltz who have both started and relieved, only the games in which they were used as a reliever were taken into account.

Here is the graph of ERA vs. Days Rest for these thirty pitchers:



As you can see, there is a definite trend between zero days rest and one day rest. In fact, 22 out of the 30 pitchers had lower ERAs on zero days of rest than on one day of rest. Further, the only two pitchers to have an ERA over 4.00 on zero days of rest were Joe Borowski and Antonio Alfonseca, both of whom have career ERAs over 4.00 as relievers. On the flip side, seven pitchers have ERAs over 4.00 on one day of rest. Overall, here are average ERAs on given days of rest among all of these pitchers (note that these are only averages of each pitchers’ ERA, not weighted averages):

Zero Days of Rest: 2.91
One Day of Rest: 3.28
Two Days of Rest: 3.24
Three Days of Rest: 3.32
Four Days of Rest: 3.01

The numbers show that pitchers perform best pitching immediately after pitching the day before. Obviously a manager cannot continuously put a pitcher out there day after day, but the fact still remains that a pitcher is best on no days of rest. After that, there is no general trend. The moral of the story is that there is not much to be said for worrying about how many days rest a reliever has when deciding whether or not to put him in the game.