ESPN NFL

Last year was a good one for a number of rookie running backs, but the two best seasons probably belonged to Joseph Addai of Indianapolis and Maurice Jones-Drew of Jacksonville.

This week, the two will square off on Monday Night Football. To get an idea of where each player's career might be headed, we decided to look at their numbers using similarity scores.

Similarity scores were first introduced by Bill James to compare current baseball players to players from the past, and the idea has spread to other sports. Similarity scores play a major role in John Hollinger's NBA analysis, and we've developed our own version that we use to look at career progressions.

Similarity scores are far from perfect, of course. They compare standard statistics like yards and touchdowns, so they are subject to all kinds of biases from strength of schedule to the quality of each player's offensive line. Nevertheless, they're fun to study and do tell you some things about players.

We've projected each player's numbers for the 2007 season based on their play so far -- adding a receiving touchdown, since each guy is likely to get one eventually -- and looked at players with similar numbers over a two-year span, as well as players who enjoyed similar rookie seasons.

Joseph Addai
Year	Games	Runs	R-Yds	R-TD	YPC	Rec
2006	16	226	1,081	7	4.78	40
2007	15	315	1526	18	4.85	41

The most similar player to Joseph Addai over his first two seasons is Terrell Davis. His yards-per-carry-average doesn't quite match, but he is very similar otherwise.

Terrell Davis
Year	Games	Runs	R-Yds	R-TD	YPC	Rec
1995	14	237	1,115	7	4.71	49
1996	16	345	1538	13	4.46	36

Earl Campbell's stats in 1979 are similar to the projection for Addai over the rest of 2007, but he ran a lot more than Addai as a rookie -- and, of course, the two players are completely different in terms of size and style.

One player does come out somewhat similar if you compare his second and third seasons to Addai's first two -- Barry Sanders. Sanders was actually younger than Addai, even though he had more experience:

Barry Sanders
Year	Games	Runs	R-Yds	R-TD	YPC	Rec
1990	16	255	1,304	13	5.11	36
1991	15	342	1,548	16	4.53	41

Other players who came out mildly similar: Willie Parker (2005-2006), Curt Warner (1985-1986), Fred Taylor (1999-2000), and Ottis Anderson (1979-1980).

If you only look at Addai's rookie season, the best comparison is not one Colts fans would get excited about. Kevin Mack averaged nearly five yards per carry for the 1985 Browns, but struggled with injuries the rest of his career and only had one other season above five yards per carry. Addai's old partner Dominic Rhodes also appears on the list of most similar rookie seasons, and considering they played in the same offense, that's not a coincidence.

Impressive Rookies
Player	Year	Team	Games	Runs	R-Yds	R-TD	YPC	Rec
Joseph Addai	2006	IND	16	226	1,081	7	4.78	40
Kevin Mack	1985	CLE	16	222	1,104	7	4.97	29
Terrell Davis	1995	DEN	14	237	1,117	7	4.71	49
Corey Dillon	1997	CIN	16	233	1,129	10	4.85	27
Terry Miller	1978	BUF	16	238	1,060	7	4.45	22
Dominic Rhodes	2001	IND	15	233	1,104	9	4.74	34

Finding players similar to Maurice Jones-Drew is extremely difficult, because he's had such a unique start to his career. Not many rookies average 5.67 yards per carry, but are unable to win a starting job. Jones-Drew is also one of only 11 players since 1978 to score double-digit touchdowns with fewer than 200 carries.

Maurice Jones-Drew
Year	Games	Runs	R-Yds	R-TD	YPC	Rec
2006	16	166	941	13	5.67	46
2007	16	173	982	10	5.69	42

Similarity scores work by starting at 1000 and subtracting the differences between players, and 52 different backs have a similarity score of 700 or more when compared to Addai's 2006 and projected 2007. Only five are that high when compared to Jones-Drew. The closest is Barry Sanders in 1989-1990, but he's not that close -- Sanders had 280 carries as a rookie, compared to just 166 for Jones-Drew.

One player does end up somewhat similar to Jones-Drew, if we look only at their rookie seasons: Ickey Woods, with the 1988 AFC champion Bengals. Woods, of course, was injured the next season and only played 21 more games the rest of his career. Here are Woods and two other players who are at least in the same ballpark as Jones-Drew:

Running Comparison
Name	Year	Team	Games	Runs	R-Yds	R-TD	YPC
Maurice Jones-Drew	2006	JAC	16	166	941	13	5.67
Ickey Woods	1988	CIN	16	203	1,066	15	5.25
Clinton Portis	2002	DEN	16	273	1,508	15	5.52
Ricky Watters	1992	SF	14	206	1,013	9	4.92

One way to see how unique Jones-Drew is would be to run the similarity scores without considering age or career experience. When we do this, five of the top six seasons belong to two players: Former Bengal James Brooks and Former Cardinal Stump Mitchell. Both players averaged 180-200 carries per season, but gained more than five yards per carry, and both players were extremely active in the passing game. A sample year for each:

Running Comparison-2
Name	Year	Team	Games	Runs	R-Yds	R-TD	YPC
Maurice Jones-Drew	2006	JAC	16	166	941	13	5.67
Stump Mitchell	1985	STL	16	183	1,006	7	5.50
James Brooks	1989	CIN	16	221	1,239	7	5.61

Just like Jones-Drew, both Brooks and Mitchell were seen as committee backs who couldn't necessarily handle a larger workload. Neither one ever had over 221 carries in a season. The difference is Jones-Drew was putting up numbers as a rookie that match what Mitchell and Brooks did in the prime of their careers, and he found the end zone nearly twice as often.

Note: The specifics of our similarity scores system are found here. It's important to note that we don't start looking for similar players until 1978, the year the 16-game season began and passing rules were liberalized.

Aaron Schatz is president of Football Outsiders, Inc. and the lead author of Pro Football Prospectus 2007 and 2008.