A Blow to Baseball

Concussions are not as big an issue in baseball as they are in boxing, football or hockey, but they are a problem. Players can collide with a wall or one another on the field or basepaths. Batters can be hit in the head with a pitch, and pitchers with a batted ball. At greatest risk are catchers, whose facemasks are routinely jarred by foul tips. Concussions account for about 2% of time lost, and are increasing at a rate of 14% per year. Although baseball has a seven-day disabled list specifically for concussion victims, there is no universal time off standard. Players can return if they show no symptoms, pass a series of physical and mental tests, and have the approval of the team doctor.

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What makes baseball attractive for study purposes is the fact that individual performance can be quantified easily. A research team head by Jeffrey Bazarian at the University of Rochester compared the performance of 66 position players (non-pitchers) who suffered concussions from 2007 through 2013 on seven offensive metrics: at bats, batting average, on-base percentage, home runs, slugging percentage, OPS (on-base plus slugging percentage), strikeouts and walks. (Of course, these seven measures are not independent.) These data were calculated for the two weeks before the concussion (pre-event), the two weeks immediately after the player's return (post-event), and the period between 4 and 6 weeks after his return (long term post-event). To control for the possibility that performance was changed by time off alone, they computed the same measures for 68 players granted leave for paternity or bereavement. The analysis statistically controlled for position (catcher vs. non-catcher) and number of days off.

The results showed significant immediate post-event declines for concussion victims on four of the measures: batting average, on-base percentage, slugging percentage, and OPS.  Overall, the players coming back from paternity or bereavement leave showed a slight improvement, suggesting there may be some value in mid-season rest. The concussion victims continued to hit more poorly relative to the leave group during the long term post-event period, but the differences were not statistically significant. Here are the means:

Batting Average

Event	Pre-Event	Post-Event	Long Term Post-Event
Concussion	.249	.227	.261
Leave	.255	.271	.269

On-Base Percentage

Event	Pre-Event	Post-Event	Long Term Post-Event
Concussion	.315	.287	.318
Leave	.331	.332	.333

Slugging Percentage

Event	Pre-Event	Post-Event	Long Term Post-Event
Concussion	.393	.347	.398
Leave	.393	.433	.404

OPS (On-Base Plus Slugging)

Event	Pre-Event	Post-Event	Long Term Post-Event
Concussion	.708	.633	.715
Leave	.724	.765	.736

The results suggest that these ballplayers had not fully recovered from their concussions before returning to action. The authors mention several possible explanations for the performance decline, including poorer visual acuity, slower reaction times, and problems with balance. Hitting a baseball thrown at 90 mph from a distance of 60 feet requires optimal performance of all these systems. However, the two measures most clearly associated with “seeing the ball,” strikeouts and walks, did not show significant change. Maybe the batters were hitting the ball at the same rate, but not hitting it as solidly.

I can understand why baseball players feel pressure to return to the field after an injury. The mean salary is slightly over $4 million, a loss to the team of almost $25,000 per game for time spent on the disabled list. This study suggests, however, that keeping the player on the bench longer might benefit not only the player but also the team.

Post Script

When this study was reported in the New York Times, the reporter interviewed Dr. Gary Green, Medical Director for Major League Baseball. The article states:

Dr. Green was . . . unimpressed with the study, which he said had major methodological problems and lacked proper controls. “You really can't draw many conclusions from it. If it shows anything it shows that the batting parameters—strikeouts and walks—are actually fairly consistent before and after injury.”

Green said he felt that there was no way to distinguish the changes the study found from ordinary variations over the course of the season that happens with all players.

There are several problems with Dr. Green's comments. Of course, no one is denying that there are consistent differences between players in batting performance. Since this study is a within-subjects design, one of its strengths is that it permits these individual differences to be statistically eliminated, allowing a more precise estimate of the effects of concussions. It was also disingenuous of Dr. Green to mention only strikeouts and walks, while ignoring the four other performance measures that are likely to be of equal or greater interest to the team management.

More importantly, Dr. Green has a fundamental misunderstanding of the logic of research design. When results are statistically significant, that means they are unlikely to be explained by ordinary variability over the course of the season. For example, if the probability of a result were less than .01, the likelihood of this result occuring by chance is less than one in one hundred. The probabilities of the four results reported in the above four tables were less than .005, .01, .004 and .003, respectively.

In my opinion, a reporter has a responsibility to do more than quote spokespersons on both sides of a controversy when one of them makes erroneous or implausible statements. Letting such remarks go unchallenged is a form of false balancing. The reporter should have pressed Dr. Green to explain what methodological problems he found with the study. He also should have briefly explained the meaning of statistical significance to the reader. If he felt uncomfortable saying these things in his own voice, he could have called one of the authors of the study and asked him or her to respond to Dr. Green.

Of course, this presumes that the reporter knows enough about research design to be skeptical of Dr. Green's remarks.

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