May 9, 2017
Do Strikeouts Spread?
The big story of the Nationals' season so far (other than that guy who got a save the other day) has been the resuscitation of Ryan Zimmerman. Zimmerman, who has battled injuries for the past few years, reached double digits in home runs for the month of April. According to a story that should probably be called “apocryphal,” Zimmerman’s renaissance can be credited to deep, late-night conversations with teammate Daniel Murphy. Murphy had one weird trick that he suggested Zimmerman might try this year: swing up. Apparently, it worked.
It’s entirely possible that Zimmerman and Murphy were talking shop in the Nationals' clubhouse. Maybe that is the reason that Zimmerman once again looks like an All-Star. It’s also possible that Zimmerman is finally healed of his injuries from the past few seasons and, while he was always trying to swing up, he’s now healthy enough to do so. Maybe it’s a little of both.
Here’s a more interesting question: Is this sort of peer-to-peer learning effect common? Do players change based on who is surrounding them? If a player goes from a team of free-swingers to a team of patient people, does he become more patient as a result? Most projection systems effectively say “No.” They tend to use a player’s stats and only a player’s stats when making a guess on what he might become in the coming season. Sure, talent is going to have something to do with it, but maybe there’s more.
Warning! Gory Mathematical Details Ahead!
Let’s pull a few tricks off the shelf. First off, we need a good measure of year-to-year change. If a batter’s strikeout rate in 2015 was 15 percent and in 2016 it was 16.6 percent, he changed in absolute terms by 1.6 percentage points. Except, well ...
Player A, 2015: 3 strikeouts in 20 PA