Kevin Pelton from the Seattle Storm wrote a nice article a few days ago looking at parity in the WNBA since 1997. His approach was to look at standard deviations of win loss records for each season.

I believe that good teams tend to play to the capabilities of their opponents, rather than to the best of their own capabilities, which allows for some artificial parity in the team records over the course of the season. For this reason I wanted to take a look at the offensive and defensive ratings in the league since 1997, and see how the spread looked from those perspectives. I am only looking at the spread between the best and worst teams in the league and the standard deviation for that year.

WNBA Offense
Offensively we are looking at a spread of the number of points scored per 100 possessions. For example, in 1997 the best team (Houston) scored 14.07 points per 100 possessions more than the worst team (Utah). The standard deviation represents a number of points per 100 possessions a team is away from the average. In 1997, the standard deviation was 4.04 points per 100 possessions. The average rating for that year was 92.27 points per 100 possessions. Houston, which had a 99.43 point per 100 possession offensive rating was ~1.7 standard deviations from the league average. Utah, which had an 85.36 points per 100 possession offensive rating was ~1.7 standard deviations from the league average.

Year	Offensive Rating Spread	Std. Dev.
1997	14.07	4.04
1998	21.51	4.61
1999	17.02	4.59
2000	25.58	6.38
2001	19.26	4.59
2002	15.66	4.11
2003	16.92	4.70
2004	10.97	2.79
2005	13.33	4.29

Offensively the league has essentially remained at the same parity it was since it was originated. The difference between the best and worst teams has changed drastically, but outside of the year 2000 when there was much less parity and the year 2004 when there was the most parity, the balance in the league has not changed much.

WNBA Defense
Here we are looking at how many points per 100 possessions a team allows. The best defensive team in 1998 was the Houston Comets, allowing only 88.22 points per 100 possessions. The weakest defense in 1998 was run by the Washington Mystics, allowing 104 points per 100 possessions. This created a spread of 15.77 points per 100 possessions between the best and worst teams defensively in 1998. The average defense in 1998 allowed 94.70 points per 100 possessions and the standard deviation was 4.40 points per 100 possessions.

Year	Defensive Rating Spread	Std. Dev.
1997	13.95	5.01
1998	15.77	4.40
1999	10.47	2.73
2000	14.50	4.14
2001	14.69	3.59
2002	13.14	3.15
2003	11.92	3.32
2004	8.91	2.76
2005	16.89	4.39

Here again we see more change in the difference between the worst team and the best, than we do in the standard deviation. There was less parity in the league defensively to start with but it moved more quickly to parity before drifting apart, returning and drifting apart once more. In general it looks like there is more parity in the league defensively, with last year representing the largest difference since the first two years of the league. 2004 is once again a year of the greatest parity with only 1999 representing a small amount more.

WNBA Differential

The differential relates to the difference between a teams offensive rating and its defensive rating. In other words, if the best team scores 105.76 points per 100 possessions and allows 92.49 points per 100 possessions (as Houston did in 1999), then it has a differential of 13.27. It will outscore its opponents by 13.27 points per 100 possessions. Compare this team to the worst team which scored of 88.74 points per 100 possessions while allowing its opponents to score 97.69 points per 100 possessions (as Cleveland did in 1999) earning a differential of -8.29. This gives the year 1999 a spread of 22.63 points in differential between the best and worst teams. The average differential for 1999 was -0.02 (it was a bad offensive year) and the standard deviation was 6.12.

Year	Differential Rating Spread	Std. Dev.
1997	22.63	7.77
1998	37.28	8.51
1999	22.22	6.12
2000	34.13	8.47
2001	19.49	5.89
2002	18.01	5.06
2003	14.88	4.69
2004	14.68	3.93
2005	20.99	6.21

Now we have something that follows Kevin's numbers in terms of trending, though the standard deviations are broader. The league shows an approach towards greater parity with 2004 offering the greatest parity. This past season has shown a wider gap, returning to the gap more like the earlier years of the league.

Is there parity in the league? For the most part, there is. What remains to be seen is whether the widening gap of 2005 is an abnormality or the start of a new trend. With the addition of the new Chicago Sky there may be some dilution of talent, and less parity than 2005, unless the influx of talent from the draft outweighs that impact. Whether the league decides to adjust its season to accomodate FIBA's conflicting schedule may also impact league parity. There is a great deal of International talent in the league, and their absence could dilute the league further and create less parity as well. For this, we shall have to wait and see.

WNBA