Monday, October 12, 2009

Expected Wins Method: Pat, Geno...and MaChelle?

A while back I wrote an article about rating WNBA coaches using a system created by John Hollinger. His idea was to find a mathematical way to predict the record of a coach before the season started and then to observe if the coach was able to achieve his or her expect win total. The coach would be graded by increase in expected wins per seasons played.

Hollinger weighted a coach's record based on

25 percent of the winning percentage two seasons before, plus
50 percent of the winning percentage one season before, plus
25 percent of a .500 winning percentage

The .500 winning percentage is a good idea: it raises the expectations for a coach performing poorly, and helps to not penalize a coach that is very successful. Both successful coaches and coaches on the hot seat should reach at least .500 every year. Coaches like Laimbeer and Dan Hughes dominated the metric, winning about 2 to 3 more games a year on the average than expected.

I then thought: if this metric worked for WNBA coaches, why wouldn't it work for women's college coaches? After all, those coaches play about 34 games a year, too. Their records are adversely impacted by injuries in the same manner as a WNBA coach. The only difference would be that winning coaches generally keep winning due to their skill at recruiting.

Furthermore, the major coaches have coached at their institutions for 20 years or more -there would be no paucity of data. I decided to choose two very successful coaches and one "test" coach who, thought generally successful, is not as bedecked in glory as my other two choices.

The first coach is MaChelle Joseph, the current coach at Georgia Tech. The two years before Joseph started, the Yellow Jackets had records of 15-14 (.517 win percentage) and 20-11 (.645 win percentage). Her predicted percentage based on the two years of her predecessor should have been (.25*.517 + .50*.645 + .25*.500 = ) .576, which translates to a 17-12 record over a 29 game season. Josephy went 14-15 instead, and had an expected win difference of -3.

We add all of the expected win differences over Joseph's eight-year career. From 2001 to the present, Joseph has gone 106-76 and has an expected win difference/season of 1.167. In general, Joseph will outpredict the formula above by 1.167 wins a season. You can think of the number - very loosely - as meaning that Joseph adds value at the rate of 1.167 wins a year.

Now, let's look at the biggest coaches of all.

Pat Summitt's started coaching in the 1974-75 season. She has eight national championships to her credit and a 1005-193 lifetime record. Her best year by Hollinger's system was the 1998-99 season where she won the NCAA Championship after coming off a 29-10 season with a 39-0 record. She had a +11 in expected wins that year.

Summitt's expected win difference/season is +2.886. That's pretty amazing, implying that three of Tennessee's wins per year are solely attributable to the coach. This year was a bad year for Summitt, who went -5 expected wins over the 2008-09 season. However, the last time she had a negative differential was way back in 1981-82, when she went 22-10. It really was the worst season for the Lady Vols in two decades.

And of course, you can't talk about great coaches without talking about Geno Auriemma. He took control of a Connecticut program that finished 9-20 in 1983-84 and 9-18 in 1984-85. His first season, he went 12-15 with the Huskies...and had never had a losing season since then.

Auriemma has had three undefeated seasons as head coach of Connecticut. He has six national championships of his own. Auriemma's expected win difference/season is an amazing +3.792. Hollinger's system says Geno is better than Pat!

We can argue with John Hollinger's methodology, of course. However, arguing between "who is better, Geno or Pat?" is like arguing "who was better, Babe Ruth or Ted Williams?" As a Tech fan, I'm hoping that someday Joseph's name will mentioned along with the greats, with the numbers to prove it.


pilight said...

The problem with using a system like that for college coaches is the extreme variability of opponents.

pt said...

True, there is a variability of opponents that could be theoretically greater than that of the WNBA's - you end up playing each team in the WNBA a certain # of times a year, even though there is some difference caused by an unbalanced schedule.

However, I think the variability would be an issue only if a team scheduled world-beaters one year and patsies the next. (Or vice-versa.) My belief - not proven - is that coaches generally schedule to the same strength year after year. Some coaches like creampuff schedules, others like to play world-beaters year after year. However, sooner or later, numbers like that would simply adjust for the quality of competition over the long run.