On Tuesday we discussed the addition of a 7-day disabled list for concussions to the MLB rules, as well as the progress being made on that front in terms of diagnoses and treatment. We mentioned that in the past, concussions had been believed to be something that happened only in the more violent NFL and NHL, due to the size, power, speed, and concussive force of the athletes involved. That is not true, though, as baseball players have suffered their fair share of head trauma in the last decade alone. Baseball players may not be able to catch a puck on the other side of the rink before icing is called or chase a halfback downfield prior to delivering the killing blow, but their knees still hurt when taken upside the head, and a fastball or liner to the cranium can do plenty of damage, too.

Our database goes back to 2003; in that stretch, there were 100 incidents diagnosed as concussions (including Yunel Escobar's from this year). Recovery from these concussions varied in length, with the shortest amount of time missed coming in at just one game (in fact, 19 of the concussions in the database caused just one game to be missed. On the high end, 108 games were lost. There are far fewer extended DL stays than short ones—after 108 comes 106, and after that the time frame drops to Justin Morneau's 78 games—but there are enough lengthy ones in there to bump up the average time lost.

As we discussed earlier in the week, the average length of time it takes for a concussion to clear is five to seven days—hence the "7" in 7-day DL. Even with just a few days missed, a concussion can mess with a player. Thanks to our database, we can evaluate the performance of every player since 2003 who has returned from a concussion and compare it to how the same players did prior to their injuries.

For pitchers, the data doesn't tell us very much. Just 11 concussions were sustained by pitchers during the time frame our database covers, and not all of them made it back to pitch within the same season. Considering that the data—in its current small-sample form—tells us that taking a serious blow to the head is actually a positive for pitchers, we're just going to go ahead and wait for more information to fall into our laps before pitchers start beating themselves up before games or relievers replace warming up in the pen with impromptu wrestling matches.

Hitters, on the other hand, have all the rest of those concussions we mentioned to check out, and therefore more intriguing data to draw upon. For our purposes, we looked solely at the concussions of players who returned to play within the same season, leaving us with 71 position players. To get a sense of how the concussions may have affected their performance once they returned, we examined how they performed for two weeks prior to their concussions, and then compared that to the two weeks that followed their return from the incident.

Sadly, we're missing quite a few concussions by looking at things this way—including those suffered by Aaron Rowand, Jason Bay, and Morneau in 2010, among others—but we're not interested in learning how well Johnny Damon did six months after sustained a concussion in October. We want to know how players have done in the short-term after sustaining and returning from their trauma.

On average, the 71 players missed just over eight games a piece, and, as a group, had 2,754 plate appearances in the two weeks prior to their injuries and 2,594 plate appearances in the two weeks following their return. The True Average for pre-concussion was the same as the league average of .260—as is expected in a sample like this, given that concussions don't care how well you hit a baseball or how many more times you can walk rather instead of striking out.

After the concussions is another story, though. Our sample posted a TAv of .254 in the two weeks of play after their return. That doesn't seem like a major difference in production, but you're talking about the difference between a league-average hitter and a league-average shortstop or catcher—in other words, the two most offense-starved positions in the game. That said, it's not so large that it's anything to worry about, either—the difference between .260 and .254 over 2,500 plate appearances (approximately the size of the post-concussion sample) is 11 runs, or one win. That is the combined dip, from all 71 players in the sample.

Playing often after returning from a concussion does not appear to be any worse for a player than taking it easy afterward. That data is pretty murky, though, given that we're talking about separate two-week samples. Travis Hafner had one of the most significant post-concussion dips in his production in 2005 after missing 17 games, but he also had two pre-concussion weeks with a TAv of .440 (in case you were wondering, Barry Bonds' career-high in TAv for a season was .433—Hafner was having himself a solid two-week stretch). A .105-point drop in TAv sounds bad until you realize that it took him down to .335. (Bonds' career TAv: .351. Even conked Pronk was mashing.)

David Eckstein took his 2006 concussion in stride, scrappily missing just three games and bumping his TAv up by .043 points in the two weeks after his return, despite the third-most PA in that post-concussion stretch of any player in the sample. Placido Polanco did not fare nearly as well—he tops the list with 68 PA in the two weeks following an injury, but saw his TAv fall from .310 to .250 after missing just the one game.

The above examples are intended to demonstrate that there is no real pattern to individual player performance after returning from a concussion. Taken as a whole, though, the sample tells us that players, on average, perform a tad worse in the weeks following a return from their concussed state, despite the best efforts of Nick Hundley (+.233 TAv in 2010), Carlos Ruiz (+.133 TAv), and others, who apparently need a knock on the noggin to get them hitting.

In some instances in our database, players may have come back before they needed to, and performed worse as a result. The good news is that with the new systems in place to make sure concussed players get the medical attention they deserve and the rest required to heal what ails them, rushing players back should not be a problem going forward. The NFL and NHL should take heed of the progress in a sport that, not too long ago, was considered relatively concussion-free.

Thanks to Dan Turkenkopf for research assistance.

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As a control, you would need to look at how hitters perform after missing ~8 games due to something other than a concussion. I have a feeling that taking time off is a generally detrimental to hitting, and might very well explain the 6 points of TAv difference.
Valid point.

Also, in the article, since the distribution of days until return from concussion seems skewed to the right, the median days until return should also be reported. The average is still potentially good to consider as a conservative estimate of how long it generally takes players to get back to playing games.
The control group depends on what we're looking at.

If we're comparing a concussion to another type of injury to see whether they're more or less debilitating, then creating a control of other injuries definitely makes sense.

If we're just looking at whether players perform worse after concussions than before, then this study does that.

Sure, other players who miss time may perform worse afterwards than before, but using them as a control is subject to selection bias.

You're generally only missing that much time if you're hurt, which means poor performance can be explained by either the layoff, or the fact that you're still not back to 100%.

It's a tough call, but I don't think this result is meaningless without the control. It just might not mean what you want it to mean.
evo34 makes the salient point. No control group = empty conclusion. Plus, I think it would be very interesting to look at season long results before and after a concussion. Are there long term implications?
I personally didn't think about using a control group based off my clinical (and stupidly not my research) experience.

In the future it's something that we are definitely going to look at.

In term of long term implications, again it's based off my clinical experience but once all of the objective measurements have returned to normal, they generally don't.