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August 7, 2009, 08:27 AM ET
Sierra and Tartabull

by Geoff Young

I was flipping through Bill James’ 1987 Baseball Abstract the other day, because that’s the sort of thing that passes for fun in my house, and came across this little nugget:

…I wrote a program to compare the 1986 rookies with those in the 1948-1975 database so as to find the ten most comparable players to each of the 1986 rookies… One way to project what a player will do is to find similar players in the past and look at what they have done… the ten comparable players will subsequently diverge… but it gives us the range of normal expectation for a player of this type.

For the crop of significant position players that were rookies in 1986 (a total of 18) James estimates their career totals based on the weighted average of the career performances of their 10 most similar players. In other words, the most similar career would be counted more heavily than the 10th most similar career, with various gradations between those two endpoints.

Inasmuch as we can predict the future from the past, and acknowledging that every individual is unique, it’s a fun toy that sometimes yields startlingly accurate results. To use a fairly trivial example, here’s how James’ estimate for Andy Allanson compares with Allanson’s actual career:

Of course, it doesn’t always work out. Barry Bonds, for example, ends up with 122 homers according to the method. (James notes that there were no truly similar players to Bonds and that such a low home run total would be a “mighty disappointment.”)

Anyway, James runs through this exercise for several rookies. It’s pretty hit-or-miss, but check out the two American League right fielders, Danny Tartabull and Ruben Sierra:

With just one season’s worth of data, those aren’t bad guesses at all. For grins, here’s how the entire crop turned out (James doesn’t provide all numbers for all players):

Well, that was fun. You know, if you like that sort of thing.

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