re: New article says Rivers having the worst season of all time

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huge caveat like "worst season of all time for a player averaging more than 25 minutes per game"

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What is WARP?
WARP stands for Wins Above Replacement Player. The term and concept are borrowed from sabermetrics and, specifically, Baseball Prospectus. Conceptually, the WARP system seeks to evaluate players in the context of a team made up of them and four completely average players. The performance of this team is then compared to that of a team made up of four average players and one replacement-level player. The method draws heavily on the work of Dean Oliver.
What are the benefits of this method?
For one, this rating system is very flexible. Players can be rated on a per-minute basis (using the theoretical "winning percentage" of the team with four average players), in terms of their offense and defense and in terms of their overall value (WARP itself). Using replacement level shows the value of players that can play heavy minutes and avoid injury while continuing to perform above replacement level. Using wins gives a measure of value that is easy to understand and constant over time. Lastly, by eschewing the traditional linear weights method so common in basketball analysis, I believe WARP does a better job of incorporating defensive value.

What are the limitations?
Like all rating systems based on box-score data, WARP cannot account for contributions that are not tracked in the box score, most notably on defense. It does no better than linear weights methods at evaluating players like Bruce Bowen. Also, it requires a number of assumptions - the value of assists, the trade-off between usage and efficiency, and replacement level.

What is replacement level? Why use it?
My colleagues at Baseball Prospectus have defined replacement level as, "the expected level of performance a major-league team can receive from one or more of the best available players who substitute for a suddenly unavailable starting player at the same position and who can be (or were) obtained with minimal expenditure of team resources." In basketball, the concept is slightly different. For example, we look at rotation players, not merely starters. Also, position is less of a factor because the nature of position is more fluid in basketball. The general principle--the production offered by the replacement for an absent player--remains the same.

Players create value for their team by playing better than replacement level. This appropriately awards credit to average players, because they are still an upgrade on freely-available replacements, while not rewarding players simply for being on the court without any level of production. It also allows us to estimate the value of players with varying playing time and productivity and account for durability.

WARP Calculated
Updated WARP statistics are available for all NBA players via the Basketball Prospectus stats pages.

A painstakingly detailed explanation of how WARP is calculated and its assumptions follows.

Offense:
The first step is evaluating a player's individual performance and efficiency. This requires calculating their points created and their possessions used.

For points, we start with points. To this, we add some credit for assists. Valuing assists is one question statistical analysis remains unable to answer with any degree of certainty, leaving us to use an estimate. I use a value of 0.75 for each assist. This is the same used by Oliver and not dissimilar from the value used by John Hollinger (.66). We also have to take away 0.75 points for each assisted field goal made to balance the ledger. While data on assisted field goals is now available at the invaluable 82games.com, to rate players from the pre-82games era on a level playing field, what I have done is used assisted field goal data to create a regression to estimate it using the share of his team's assists the player distributes, the team's assisted field goal percentage, the player's usage percentage, his offensive rebound rate and the percentage of his shot attempts that are three-pointers.

The complete formula is: TmAst/TmFGM * (1.53 - 1.442 * [Ast/Min / (TmAst/(TmMin/5))] -0.041 * ((OReb/Min)*48) - 0.787*Usage + 0.014*(3A/FGA)^2*(1/Usage)^2)

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