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Further Explanation of NCAA Plus-Minus
January 24th, 2010 by Jon Nichols

I think there’s been some confusion about the college basketball plus-minus I’ve been presenting here lately, so I’d like to provide some more information explaining what the numbers mean exactly.

The most important thing to realize is that the offensive, defensive, and overall plus-minus numbers are “net.”  This means every player is essentially compared to only his teammates and not the rest of Division I.  Therefore, the worst team in the country can have just as many positive plus-minuses as the best team.

Why does it work this way?  The plus-minus I use is on/off.  That means that each rating is a comparison of how the team does when a given player is on the court versus how they do when he is off the court.  For example, if Kentucky scores 80 points per game while John Wall is on the court and 75 points per game while he is off the court, his net offensive plus-minus is 5 (80-75=5).  If they allow 60 points per game while he is on the court and 65 while he is off the court, his net defensive plus-minus is -5 (60-65=-5).  As you can see, it’s better to have a positive offensive plus-minus and a negative defensive plus-minus.  Overall net plus-minus is simply the combination of offensive and defensive plus-minus.

Hopefully it is clear now why really good teams have a lot of players with negative plus-minuses.  If not, here is a real example.  Eric Bledsoe of Kentucky has a -10 net overall plus-minus.  Is he a bad player?  Absolutely not.  Remember that his plus-minus is simply a comparison of Kentucky’s production while he is playing to Kentucky’s (and only Kentucky’s) production while he is not playing.  It is safe to say that Kentucky is quite good even without Eric Bledsoe.  So he’s being compared to a very high standard.

This leads me to my next point.  Plus-minus is very unreliable without a large data set (in this case, games played) to work with.  I have made this point many times, but it bears repeating: take these early plus-minus numbers with a grain of salt.  Things can and likely will change as the season progresses.

Two more clarifications: the first three columns (net overall, net offense, and net defense) are per an average-paced game, which is estimated to be 66.5 possessions.  Finally, the column “plus-minus total” is simply the sum of the points a player’s team scored while he was in the game minus the points they allowed while he was in the game.  For those unfamiliar with on/off ratings and unsure why good teams have players with negative net plus-minuses, this column may be what you thought you were looking at.  If you notice, just about every player on good teams has a positive plus-minus total.  Again, this is a measure of how that player’s team outscored their opponents over the whole season while that player was in the game.

If you have any more questions, don’t hesitate to ask.  Just leave a comment below the post.  Thanks!


7 Responses  
Alex writes:
February 10th, 2010 at 1:23 pm

Jon,

You make a valid mention of sample size issues. However, there is also the issue of sample size for players with extreme values of minutes percentage. For example, a player that plays 90% of available minutes leaves data for the comparison from only 10% of the time, and these are usually garbage time minutes that may be less of a reflection on the player’s value. But it’s certainly good data, and a nice job compiling and presenting it.

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