Jeff Bagwell is to James Loney as Pedro Martinez is to Chad Billingsley?

Before there were such things as CHONE, Pecota, and Bill James' annual projection for players, there was a charming little thing called Similarity Scores, also created by Bill James.  If you look at baseball-reference.com, you will find a description of the formula used for similarity scores and a credit to James' book The Politics of Glory; however, I first read about them in The Bill James Baseball Abstract 1986, in which he introduced the concept, having worked on it over the past year.  This was also the book that introduced me to the term sabermetrics, among other things.

I stumbled across The Bill James Baseball Abstract 1986 while looking in a book store for a book that contained all the major league statistics for the 1985 season (no baseball-reference.com in those days), which I needed to prepare for a Strat-O-Matic baseball draft.  (I went with a HR friendly ballpark and loaded up on power, defense be damned, and drafted Pedro Guerrero #1 at 3B/1B.)  The book was an eye-opener in many ways and made me aware that there were a lot of ways to look at baseball and baseball statistics than just the traditional versions.  I wonder if I would have even gravitated toward the baseball blogs that I enjoy so much today had I never crossed paths with James' Abstract. By the way, in the Dodgers chapter, James defends Lasorda's decision to pitch to Jack Clark.

If you didn't read the formula in the link, the brief version is that 1000 is a perfect match between two players.  Points are then deducted from there based on differences in position, a bunch of counting statistics (e.g., hits) and a few ratios (e.g., batting average, ERA).  Obviously, the higher the number, the better the match.

At baseball-reference.com, they calcluate the similarity scores on each player's page, giving the top-10 career similarities, plus the top-10 at each age of the player's career, near the bottom of the page.  For example, if you scroll toward the bottom of ex-Dodger Shawn Green's player page, you'll find that his number one match in career similarity score is ex-Dodger Reggie Smith at 945.  This is what similarity scores (using this formula) seem to work best for, evaluating player production over an entire career, which is good for things like Hall of Fame arguments.  You'll also see that at age 29, after his last great season (2002), the 29-year old he was most similar to was Dale Murphy with a similarity score of 942.

Similarity scores don't make for a particularly great projection system, because, among other things, the stats used aren't the best for that purpose and the similarities don't account for trends (i.e., improving or not), but I'd bet that Bill James used similarity studies with different stats in play to fine tune his projection system.  However, that doesn't stop the age-based similarity score summaries at baseball-reference.com from being a lot of fun to peruse and to compare current players to players throughout baseball history and see how that current player might turn out.  This turns out to be a fine time to do such a thing because a) the season is over and the hot stove is still thawing out, and b) baseball-reference.com just updated them all to include the 2009 season data.

After the jump are similarity score summaries for the selected returning Dodger players at their 2009-season age.  These show the top ten similar players at the same age - some of the names may surprise you:

UPDATE - corrected lists for Russell Martin, Rafael Furcal, and Casey Blake. The previous lists were incorrect due to a bug regarding position adjustments in the baseball-reference.com software that was present at the time this article was originally written. I left the incorrect tables below as well, just for the curiosity of seeing things like Bobby Grich in Russell Martin's list.
Corrected Tables
Russell Martin, Age 26 Casey Blake, Age 35 Rafael Furcal, Age 31
Similar Player Similarity Score Similar Player Similarity Score Similar Player Similarity Score
Thurman Munson 966 Aaron Boone 938 Craig Biggio 934
Sammy White 956 Melvin Mora 930 Jay Bell 921
Ray Fosse 942 Jim Hickman 911 Tony Fernandez 910
Ed McFarland 941 Dave Hollins 910 Ray Durham 900
Rich Gedman 933 Charlie Maxwell 909 Barry Larkin 895
Yadier Molina 932 Jim Lemon 908 Cristian Guzman 893
Benito Santiago 932 Mel Hall 907 Herman Long 887
Jack Clements 930 Hoot Evers 905 Orlando Cabrera 885
Tim McCarver 926 Jay Payton 903 Lou Whitaker 885
Bob O'Farrell 926 Kevin Millar 902 Frankie Crosetti 885

Original Tables
 Similar Player Similarity Score Similar Player Russell Martin, Age 26 James Loney, Age 25 Rafael Furcal, Age 31 Thurman Munson 971 Jeff Bagwell 975 Craig Biggio 944 Felipe Lopez 966 Ted Kluszewski 974 Ray Durham 940 Mike Andrews 963 Alvin Davis 960 Jay Bell 921 Sammy White 962 Joe Hauser 948 Lou Whitaker 920 Ray Durham 960 Babe Young 943 Jason Kendall 920 Lonny Frey 960 Rafael Palmeiro 939 Delino DeShields 916 Gil McDougald 960 George Kelly 939 Chuck Knoblauch 915 Aaron Ward 954 Steve Garvey 939 Brian Roberts 914 Bobby Grich 953 Joe Adcock 936 Tony Fernandez 910 Barry Larkin 951 Adrian Gonzalez 935 Paul Molitor 909
 Similar Player Similarity Score Similar Player Casey Blake, Age 35 Manny Ramirez, Age 37 Juan Pierre, Age 31 Aaron Boone 938 Ken Griffey, Jr. 857 Willie Wilson 943 Melvin Mora 930 Jimmie Foxx 849 Clyde Milan 916 Jim Hickman 911 Frank Robinson 840 Max Carey 910 Dave Hollins 910 Rafael Palmeiro 813 Duff Cooley 908 Charlie Maxwell 909 Barry Bonds 812 George Case 904 Jason Varitek 909 Gary Sheffield 811 George Burns 903 Jim Lemon 908 Jim Thome 811 Steve Brodie 896 Mel Hall 907 Jeff Bagwell 810 Ginger Beaumont 892 Hoot Evers 905 Chipper Jones 803 Harry Hooper 884 Jay Payton 903 Willie Mays 800 Mickey Rivers 873
 Similar Player Similarity Score Similar Player Matt Kemp, Age 24 Andre Ethier, Age 27 Clayton Kershaw, Age 21 Nick Markakis 971 Richie Zisk 971 Britt Burns 972 Carlos Beltran 961 Bobby Higginson 970 Moe Drabowsky 971 Vernon Wells 960 Sam Chapman 966 John Henry Johnson 971 Enos Slaughter 955 Aubrey Huff 962 Jack Fisher 970 Ellis Burks 954 Jacque Jones 961 Carl Cashion 970 Bobby Bonds 945 Bill Nicholson 959 Ken Holtzman 969 Ellis Valentine 944 Jim Edmonds 957 Norm Baker 968 Rocco Baldelli 944 Jason Giambi 957 Jerry Walker 966 Bug Holliday 941 Larry Doby 953 Dontrelle Willis 966 Andre Dawson 938 Reggie Sanders 951 Mike Witt 965
 Similar Player Similarity Score Similar Player Chad Billingsley, Age 24 Hiroki Kuroda, Age 34 Jonathan Broxton, Age 25 Ray Culp 977 Masato Yoshii 966 Doug Bird 961 Pedro Martinez 975 Sig Jakucki 959 Chad Cordero 958 Jake Peavy 973 Walt Dickson 957 Roger McDowell 958 Stan Williams 970 Kenshin Kawakami 950 Huston Street 956 Sid Fernandez 966 Frank Oberlin 949 Victor Cruz 956 Carlos Zambrano 965 Jimmy Zinn 948 Steve Bedrosian 953 Dennis Martinez 965 Lou Tost 946 Antonio Osuna 953 Scott Kazmir 963 Art Delaney 946 Luis DeLeon 953 Dave McNally 961 Bob Keegan 945 Scott Williamson 951 Ramon Martinez 959 Tom Filer 944 Tom Niedenfuer 950
 Similar Player Similarity Score Similar Player Hong-Chih Kuo, Age 27 George Sherrill, Age 32 Ramon Troncoso, Age 26 Hank Aguirre 982 Ricardo Rincon 967 Scott Sullivan 988 Rich Folkers 981 Pedro Feliciano 962 Jack Wisner 985 Gene Walter 978 Rich Rodriguez 960 Gary Wagner 984 Juan Agosto 977 Tony Castillo 956 Roy Thomas 984 Dick Kelley 976 Steve Frey 950 Jack Billingham 984 Greg Cadaret 975 Kerry Ligtenberg 949 Brandon Medders 982 Rafael Perez 975 Graeme Lloyd 949 Bobby Locke 982 Ken Frailing 975 Tom Hilgendorf 946 Hersh Freeman 981 Pedro Martinez 975 Mike James 943 George Frazier 981 Dave Leiper 974 Jim Hughes 942 Toby Borland 981

Did I acknowledge baseball-reference.com enough?

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