Though Stephen Strasburg won't begin the year in the majors, he won't be down on the farm for long. So once he comes up, what can we expect from him as a rookie?
Last year, the best rookie pitcher was Atlanta Braves right-hander Tommy Hanson, who had a sterling 2.89 ERA after facing 522 batters. But each year, we should expect a handful of guys to pitch that well in their “pure” rookie season. The pure rookie season is the year they first starting playing in the majors, no matter how short the stint. Since 1993, 1,585 pure rookies have faced at least 27 batters in their first year. And, as expected, they’ve struggled. The group has allowed runs at a rate of 123 percent of the league average (RAindex), for an average 5.48 ERA. (RAindex is the percentage calculated by dividing runs a pitcher allows by the league average. So the average pitcher has an RAindex of 100.) The top pure rookies -- the 90th percentile -- fare much better with an RAindex of 61. The worst rookies, the bottom 10th percentile, had an RAindex of 194. That list includes pretty much everyone who got a cup of coffee, so let's increase the standard a bit.
What if we bump up the batters faced to 250? That's 378 pitchers, with 90th and 10th percentiles of 71 percent and 136 percent, respectively. The mean is 104 percent.
Finally, let’s bump up the batters faced to 500 or more. That's 97 pitchers, the top 10th percentile allowing 78 percent the league average and the bottom 10th percentile allowing 124 percent. The mean is 101 percent.
What does it all mean? You should expect 10 percent of pure-rookie pitchers to post an ERA that is somewhere at the 60 to 80 percent of league average, or better, depending on where you set the minimum batters faced level. So if the league ERA is 4.30, expect 10 percent of pure-rookie pitchers to post an ERA of 2.62 to 3.35, or better. For some context, check out a list of last year's rookie arms to see how they fared relative to the average.
Pure Rookies Pitchers, 2009
RAindex is the percentage calculated by dividing runs a pitcher allows by the league average. So the average pitcher has an RAindex of 100. (Minimum 500 batters faced.)
In terms of talent, we know Strasburg will be in the 90th percentile of rookies. Soon we'll get to see if he puts up the numbers to match.
Tom Tango writes for Inside The Book.