APBRmetrics

Mike G · Posted: Mon May 24, 2010 8:18 am Post subject:

Schtevie, I believe the thought process is this:
NBA teams are forced to use below-average players for many minutes per season. While we all agree certain guys are below average, the debate is about whether they're actually hurting their teams' ability to win.

If teams are using players who are below the value of available replacements, then it's arguable that the use of these players actually does hurt them. If they're the best available, and you have to play someone, then they're as good as it gets.

In another thread, DSMok1 chose to evaluate the wins contributed by players at various draft positions, using [WS/48 - .025] . In other words, wins above replacement, defined as 25% of average win-production rate.

Since we find players such as Monta Ellis getting 41 mpg, at a rate of .023 WS/48, it's a wonder that legendary coach Don Nelson would be inclined to 'hurt' his team by playing a guy who adds nothing to the team's win probability, and who could be easily replaced.

If we have an educated estimate of replacement value -- perhaps .50 , .20 , or -.10 of average -- it helps us form statements about a player's real 'value'.
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DLew · Joined: 13 Nov 2006 Posts: 224

Mike,

I don't think anyone would argue Monta Ellis adds nothing to his team's win probability in absolute terms. Clearly having him out there is better than having only 4 players on the court. I agree only with the second half of your statement, that (according to Win Shares) he adds nothing above what a freely available replacement player (assuming the 0.025 threshold) would add. Interestingly enough, the Warriors employed several freely available replacement players this season (Anthony Tolliver, Coby Karl, Reggie Williams). Tolliver and Williams rated somewhat higher than Ellis, Karl not so much.

DSMok1 · Posted: Mon May 24, 2010 10:04 am Post subject:

Here's what a graph of Win Shares over Replacement vs. MPG vs. Team Efficiency Differential looks like, using the 0.025 level, historically:

As you can see, a league average team basically doesn't play anybody that's below replacement level. However, as soon as you get below league average, a few players of that type start to play. Once you get down into BAD territory, then a lot of the replacement-level players start to appear.

Mike G · Posted: Mon May 24, 2010 10:53 am Post subject:

DSMok1 · Posted: Mon May 24, 2010 2:24 pm Post subject:

DLew · Joined: 13 Nov 2006 Posts: 224

I should elaborate on the point I was trying to make: Replacement level is a useful construct for assessing player value because absolute zero is so difficult to assess. Value over replacement is much easier to determine than just 'value' because replacement level performance exists in the NBA, absolute zero level performance has never existed in the NBA so it's extremely difficult to calculate where it might fall.

Generally I think value over replacement is useful to explicitly calculate because every rating system is a value over something, it's just those that don't explicitly define zero level to some meaningful mark are much harder to interpret.

schtevie · Joined: 18 Apr 2005 Posts: 411

DSMok1, I was hoping for more persuasive arguments as to the utility of the concept of a replacement player in NBA basketball.

It surely is possible that there is a potential supply of such players numbering in the hundreds or thousands. But this is ultimately irrelevant. The demand for such players is tiny, and ever shall it remain so. And because the market is so thin, there is no real reason to reify this concept. To the contrary, it only creates confusion with no attendant benefits.

Who are replacement players? Basically, they are practice bodies and insurance policies. They play when there is injury, garbage time, and for a few minutes here and there for the benefit of team cohesion.

Why is it desirable to establish the "average" performance of such players? Surely not as a benchmark for those mostly outside this group. This isn't baseball, where because of the rules of the game, the worst players basically get as many at bats as the best. And, this aside, it isn't meaningful to speak of a single replacement level anyway. Eli W's clearly showed that each position has its own average.

To beat a horse that should die, basketball is not baseball. Basketball needs improved estimates of player values, not arbitrary referential standards for the worst players.

HoopStudies · Posted: Mon May 24, 2010 4:56 pm Post subject:

Replacement player value is useful because everything we do is about making a decision, usually a cost vs a benefit. Costs are usually straightforward - a player salary, though negotiation of that at times is the issue. Benefits are harder - the value of a win and the responsibility of a player for those. If you're going to pay a lot of money for a player, you should want to know their value, especially relative to the lowest price player out there. Do you buy generic if you think it's just as good as Frosted Flakes because it costs less? Or do you assume Frosted Flakes are better because they cost more? When our decisions value a lot more than Frosted Flakes, we want to know how good both the original and the generic are. In our case, there are a lot of essentially "generics" available every year. Assigning a general replacement value saves you from having to really value them all, which can be tricky with their limited minutes.
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mtamada · Joined: 28 Jan 2005 Posts: 377

Yup, replacement level is crucial in making roster decisions. Even if we ignore salaries, we still have to compare player qualities somehow, which means making tradeoffs between high performance, low minute players versus lower performance but higher minute players -- KevinP's 2004 comparison of Shaq vs McGrady is an excellent one. Ignoring for the moment salary and what the teams' needs are, i.e. team fit, would you trade one of those players for the other? Which was more valuable, McGrady's higher points, assists, etc. in 2003, or Shaq's higher per-minute productivity? The correct baseline to compare those players against is the replacement level player, not the average player (even the Lakers, Celtics, and Magic don't have a marginal player who is at league average level, rather their marginal player is way below that level).

People who ignore replacement level will overvalue the longer minutes but lower performing player. We see this all the time in Hall of Fame debates, where all too often the arguments are made based on the accumulation of stats such as points and rebounds, and even per game statistics, and too little attention is paid to the player's performance level. (The problem is even worse in baseball where some people will point at Pete Rose's 4,256 hits, and fail to take into account how many outs he made in accumulating those hits.)

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HoopStudies · Posted: Tue May 25, 2010 8:12 am Post subject: