Met/Yankee Power Number

Bill James used the mathematical concept of the "harmonic mean" to create what he called the "power-speed" number, identifying the players with the most substantial combination of home runs and stolen bases. You get the harmonic mean of two numbers by dividing double the product of the two numbers by the sum of the two numbers. A 30-30 homer-SB season produces a harmonic mean "power-speed number" of 30, while a season with 60 homers but only 1 stolen base, or a season of 60 SBs but only 1 HR, produces the much lower power-speed number of about 2. The point of using the harmonic mean formula is that you need high numbers in both categories to get a high combined number.

We can use the same harmonic mean formula to see which players have hit a bunch of homers for both the Mets and the Yankees. Using HRs for the Mets and HRs for the Yankees as the two data points to get a harmonic mean "Met-Yankee power number" for hitters, here are the top 10 guys in "Met-Yankee power number"

1. Darryl Strawberry 70.5
2. Robin Ventura 49.1
3. Rickey Henderson 20.8
4. Tony Clark 16.0
5. Charley Smith 15.7
6. Marv Throneberry 15.5
7. Claudell Washington 14.4
8. Lee Mazzilli 11.0
9. Todd Zeile 10.5
10. Gene Woodling 9.1

Altogether, 34 guys have hit at least one homer for each of the Mets and the Yankees. The full list of players who have played for both the Mets and Yanks is here: http://www.baseball-reference.com/friv/multifranchise.cgi?level=franch&t1=NYM&t2=NYY&t3=&t4=&submit=Find+Players

Marv Throneberry, whose name has become a kind of synecdoche (look it up!) for the hapless early Mets, is in 6th place on the Met-Yankee power number list. It is not often mentioned that Marvellous Marv spent his first three major league seasons with the Yankees: he played in 141 games as a Yankee but only 130 for the Mets, had almost as many ABs for the Yankees (344) as for the Mets (371), and had very similar homer and BA totals for the two teams (16 HRs and .240 BA for the Mets, 15 HR and .238 BA for the Yankees).