The logic behind my RPI ratings and why the results are sometimes counterintuitive

There have been legitimate questions raised about my "Relative Performance Indicator" ratings I have been using to evaluate the game performance of the Milwaukee Bucks. Let me explain why the questions have arisen, why I went with the RPI method, and why I still believe it is the best method for evaluating the performance of each of the Milwaukee Bucks.

The questions have arisen (as I hoped they would... it means people are scrutinizing the information I provide) because of some queer results the RPI method has produced. (Click here to see the RPI results for last night's Bucks game). Specifically, last night Bucks center Andrew Bogut recorded what would ordinarily be a horrible Win Score 48 for a center: 8.5 (NBA center's normally average around 11.5-12.0). Yet, by RPI standards his performance was deemed "above average" (his RPI was a +0.349). The obvious response to that rating is... WTF??? He played like crap, didn't he?

The answer is: compared to the numbers other centers have produced against the Phoenix Suns this season, he actually played fairly well. To explain why that is, and why -- as strange as it is -- it has value, I have to briefly explain why I decided to go with a Relative Performance Rating in the first place.

At the beginning of the season I was comparing each Bucks performance against last season's averages. But doing some calculations, I noticed positional Win Scores in the NBA tend to fluctuate from season to season. Thus when in-season, I have no solid NBA-wide averages to reliably compare to the Bucks performances. And early in the season that discrepancy is magnified. The Bucks obviously play a limited mixture of Association teams until well into the scheule, so it really makes no sense to compare them against any existing Association-wide averages until the season's over.

So instead of doing that, I decided the best way to provide Bucks fans with a solid evaluation of how each Buck is doing at any given point in the season is to compare each Bucks performance against the performance turned in by the rest of the Association against the same opponent. Its the same basic thinking used by FootballOutsiders.com when they do their analysis of each NFL players game performance. Basically, they believe the best method for providing a neutral evaluation of the strength of any given player's performance is to ask how that player did compared to how the rest of the NFL did against the same opponent.

And I think it makes sense. Let's say you're John Hammond and you sign a guy to a ten day contract. And let's say within those 10 days you play some of the worst defenses in the Association. And lets further say the guy puts up huge stats against those weak opponents. After the 10 days expire, you have to decide whether to extend the guy. When doing so, are you going to take his 10 day stats at face value, or will you more likely adjust them to account for the lesser competition? I think it makes sense to adjust them. By doing so, you can project more accurately how the guy is likely to perform against the rest of the Association.

And that's all RPI does. It simply adjusts a player's statistical production in accordance with the production of the rest of the Association against that opponent.

So you're going to get some funny results early on. It just happened that the Phoenix Suns have been shutting opposing centers down to date. And it further happened that Bogut produced better statistics than the other centers have been able to produce... however weak those statistics may have been in absolute terms.

But I think, as weird a result as that was, it was still informative. Phoenix has been lockdown against centers. Bogut outproduced those other centers. Even though the sample size for Phoenix's defense has been small, doesn't Bogut's RPI suggest he may be able to outproduce other centers in other situations as well? After all, the Suns have probably faced about 12 centers prior to Bogut. Why couldn't those 12 produce the measly 8.6 WS48 Bogut produced? Was Bogut simply better?

That's all I'm trying to figure out. I'm trying to give Bucks fans an opponent-neutral evaluation of how each Buck is performing -- and how their defense is performing on each opponent -- on a game-by-game basis.