Earlier this week, I posted a link to a study published in the Milwaukee Journal-Sentinel that looked at the changes in performance by players discussed in the Mitchell Report. Frank Stephenson took the study to task for not properly interpreting the data.
In today’s New York Times, two professors with strong backgrounds in statistics, Jonathan Cole (sociologist, Columbia) and Stephen Stigler (statistician, University of Chicago), report their analysis of players mentioned in the Mitchell Report.
For pitchers identified by the report, we looked at the annual earned run average for their major league careers. For hitters we examined batting averages, home runs and slugging percentages. We then compared each player’s yearly performance before and after he is accused of having started using performance-enhancing drugs. After excluding those with insufficient information for a comparison, we were left with 48 batters and 23 pitchers.
For pitchers there was no net gain in performance and, indeed, some loss. Of the 23, seven showed improvement after they supposedly began taking drugs (lower E.R.A.’s), but 16 showed deterioration (higher E.R.A.’s). Over all, the E.R.A.’s rose by 0.5 earned runs per game. Roger Clemens is a case in point: a great pitcher before 1998, a great (if increasingly fragile) pitcher after he is supposed to have received treatment. But when we compared Clemens’s E.R.A. through 1997 with his E.R.A. from 1998 on, it was worse by 0.32 in the later period.
Hitters didn’t fare much better. For the 48 batters we studied, the average change in home runs per year “before” and “after” was a decrease of 0.246. The average batting average decreased by 0.004. The average slugging percentage increased by 0.019 — only a marginal difference. So while some batters increased their totals, an equal number had falloffs. Most showed no consistent improvement, several showed variable performance and some may have extended the years they played at a high level, although that is a difficult question to answer.
This confirms Stephenson’s simple analysis. I’m sure it will be easy to find quibbles and possible alternate explanations for these results; but please, keep in mind that the authors are limited by what they can say in 800 words.
Aside: Stephen Stigler is the son of Nobel Prize winning economist George Stigler.
Thanks to Repoz for the pointer.