Do We Overvalue Rebounds?

About six weeks ago King Kaufman – the sports writer at Salon.com – wrote a column that criticized our work in The Wages of Wins. Kaufman’s work has a number of problems, some of which I will examine in subsequent postings. Today I want to focus on Kaufman’s claim that “some people are vastly overvaluing rebounds.”

I think the “some people” are us, so let’s examine what the analysis of “some people” (us again) says on this topic.

Let’s begin with NBA Efficiency, the metric cited approvingly by Kaufmann. The NBA Efficiency metric is calculated as follows:

NBA Efficiency = Points + Rebounds + Steals + Assists + Blocked Shots – All Missed Shots – Turnovers

Compare this metric to Win Score, the simple measure we introduce in The Wages of Wins.

Wins Score = Points + Rebounds + Steals + ½*Assists + ½*Blocked Shots - Field Goal Attempts – ½*Free Throw Attempts – Turnovers – ½*Personal Fouls

If we look at these two measures, what differences do we observe? Relative to points scored – the factor that dominates player evaluation in the NBA – both measures value rebounds, steals, and turnovers the same. Let me repeat, the NBA Efficiency measure – again which Kaufman quotes approvingly in his column – values points, rebounds, steals, and turnovers the same.

So what’s the difference? One can note that Win Score has different values for assists and blocked shots. One can also note that we incorporate personal fouls. But these are not the truly important difference. The important difference is how we value shooting efficiency.

As we observe in Chapter Seven of our book, the NBA Efficiency metric fails to emphasize the importance of shooting efficiency. One can read the relevant excerpt HERE.

For a slightly shorter version of the story, consider a player who takes three shots from two point range and makes one. According to NBA Efficiency this player has broken even – adding two from the made basket and losing two from the missed shots. So with a shooting percentage of 33% this player breaks even. If he shoots better than this meager threshold – and no NBA player who played at least 500 minutes last year failed to at least equal this minimum – the more he shoots the higher his value in NBA Efficiency. From three point range the break even point is only 25%. Again, if you shoot better than this rate from three point range, the more you shoot the better your value according to NBA Efficiency.

The proper valuation of shooting efficiency is THE key difference between the NBA’s metric and our measure. As we note in our book, even if you are not a very efficient shooter – like Antoine Walker or Allen Iverson – you will still rank fairly high in NBA Efficiency. All a player needs to do to secure a high ranking is take an above average number of shots. One should note that these players are still paid very well, further evidence that shooting efficiency is also not properly noted in the salary determination process in the Association.

Kaufman notes that “It’s part of every basketball player’s job to score, pass, rebound and play defense.” Yet the metric he praises is dominated by only one aspect of performance, scoring.

Let’s go back to Kaufman’s comment on rebounds. To illustrate how rebounds are not the factor that dominates Win Score, I created a Kaufman-inspired adjusted formula. Specifically, I kept everything the same but I lowered the impact of a rebound to only 70% the value of a point.

In other words, I created the following formula:

Kaufman-inspired Wins Score = Points + 0.7*Rebounds + Steals + ½*Assists + ½*Blocked Shots - Field Goal Attempts – ½*Free Throw Attempts – Turnovers – ½*Personal Fouls

The correlation between Win Score and the Kaufman inspired formula is 0.99 for players in the 05-06 season. In other words, changing the value of rebounds does not change your results. Clearly, the issue is not rebounds. Oh, and no I am never using the Kaufman-inspired formula in any analysis. I just created this to make a point.

Two more points…the same criticism of NBA Efficiency also applies to John Hollinger’s Player Efficiency Ratings (PER). This is a point I will elaborate on in the future. For now it is interesting that two metrics that note efficiency in their titles do such a poor job of evaluating inefficient scorers.

And, as I noted many times before, it is important to note that Win Score is the simplified version of Wins Produced. It does not take into account the tempo a team plays or the quality of its defense. And it doesn’t adjust for position played. All of these issues are noted in the calculation of Wins Produced.

As I mentioned above, the inability to recognize the relative value of rebounding was not the only problem with Kaufman’s analysis. Next week I will discuss how his analysis reported in his column fails to accurately measure the link between rebounds and wins.

- DJ

Our research on the NBA was summarized HERE.

Wins Produced and Win Score are Discussed in the Following Posts

Simple Models of Player Performance

Wins Produced vs. Win Score

What Wins Produced Says and What It Does Not Say

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