Monday, August 31, 2009

Rating WNBA Coaches: The Expected Wins Method

One of two challenges given to me by the author of the Rethinking Basketball blog was to rank which coaches have been the best coaches of the WNBA based on the method used by John Hollinger.

Hollinger - who is mysteriously listed as "Dave Hollinger" in the article - uses a very simple method. Figure out how many games a team should win and compare the actual number of wins to that number. If a team exceeds the expected wins, we credit those wins to the coach. If the team fails to meet the mark, we debit the coach.

The first problem becomes determining the number of expected wins for a team. Hollinger uses the following method:

expected winning percentage for a team = (0.25)*(winning percentage of season before last) + (0.5)*(winning percentage from last season) + (0.25)*(.500)

Note that 1/4 of the formula above is based on a .500 record. This serves two purposes - to give the coach of a sub-.500 team something to strive for, and to give the coach of a winning team a reward for continuing to win.

Hollinger examined all coaches that had five complete seasons of experience. However, the WNBA presents a problem with its multiple expansions. The formula does not handle the case of a coach coaching in the first or second year of a franchise's existence. It's probably a good idea that the formula fails, since it's more difficult to judge a coach given the dearth of talent in those first two years.

For example, Marynell Meadors loses three complete years - the first two years with the Charlotte Sting in 1997-98 and the first year with the Atlanta Dream in 2008. Van Chancellor's first two years with the Houston Comets can't effectively be judged as Houston was an "expansion team". Who is to say that he did a great job? Maybe the talent just fell into his lap.

Therefore, the list of coaches below:

a) have coached five complete seasons in the WNBA, where
b) any season which is one of the first two seasons of a new franchise isn't counted.

This gives us six coaches to look at who meet the criteria: the following lists the coaches name, the total number of wins above expected over the number of seasons evaluated, the number of seasons evaluated, and the wins above expected per year.

Hughes 23 7 3.29
Laimbeer 18 6 3.00
Donovan 13 6 2.17
Thibault 8 5 1.60
M. Cooper 9 6 1.50
Chancellor 0 8 0.00
Adubato -1 8 -0.13

For example, let's look at the leader, Dan Hughes. Hughes started out by finishing Marynell Meador's 1999 season with the Charlotte Sting where Meadors was fired midway through. We won't count that season. Hughes was let go by the Sting after that season anyway, replaced with T. R. Dunn.

Hughes then shows up in Cleveland in 2000 and coaches his first complete season. The franchise has now existed for at least two years, so we can use the formula.

expected wins for Cleveland Rockers in 2000 = (0.25)(1998 win percentage) + (0.5)(1999 win percentage) + (0.25)(.500 win percentage) = 0.40125.

The Rockers played 32 games in 2000, which means that the Rockers were expected to win 32 * 0.40125 = 12.84 games, or 13 games. Hughes won 17 games that year, giving him a +4 in expected wins.

Now let's look at Van Chancellor. He started coaching with the Comets in 1997, but we can't start evaluating his seasons until 1999 since the formula doesn't work for any season which is one of the first two in the franchise history.

expected wins for Houston Comets in 1999 = (0.25)(1997 win percentage) + (0.5)*(1998 win percentage) = (0.25)(.500 win percentage) = 0.73575.

The Comets played 32 games in 1999, which means that the Comets were expected to win 32 * 0.73575 = 23.544 games, or 24 games. Chancellor won 26 games that year, giving him a +2 in expected wins.

(* * *)

Do the results meet the smell test? In other words, if I had asked you to order these coaches from best or worst, would the answers be what you expected?

Seeing Hughes and Laimbeer in the 1-2 positions might satisfy some readers, but others might think that Laimbeer belongs on top. Donovan, Thibault and Michael Cooper at least have positive values in expected wins per year, lending credence to the belief that those three coaches are at least better than average. (Some Sparks fans might dispute that about Cooper, though.)

Van Chancellor coming out with zero expected wins above average per year is a bit shocking. Was the Comets dynasty more a function of its players than its coach? Chancellor doesn't get any credit in the metric for the first two years of the Comets performance. Richie Adubato is the only coach on the list with a negative value in expected wins per year, and both he and Chancellor are the only coaches on the list that are no longer coaching.

1 comment:

pilight said...

I'm not fond of that system generally, but especially for the shorter seasons in the W. Injuries to players have a greater impact when there are fewer games in the season. A big star missing 10 games in an NBA season is bad but can be overcome, in the W it's all but a season ender.