What is the value of an assist? Part I
The first big issue I’d like to tackle is to consider what the actual value of an assist is. In a lot of player rankings, they give an assist the value of 2 points, since it’s a pass that leads to a bucket for your team. But this is obviously wrong, since even if you didn’t make the pass that led to the assist, your team would still have a good chance to score. So it didn’t add a value of two points to your team. This gives us a first cut at determining the value of an assist. Since the average points scored per possession is 0.876, then an assist adds a value of 1.124 points to your team (2 points for the basket off the assist minus the average points the team would otherwise have scored, 0.876.)
But it’s actually worth more than this, since that 0.876 number includes all the baskets with assists. If you assume that all baskets with assists are sure scores (FG% of 100), then you have to subtract those off to determine the average number of points scored without an assist. The average NBA team shoots 80.6 times per game, making 35.68 shots, with 21.08 assists. Pulling out the assists leaves the team making only 14.6 shots out of 59.52 attempts, for a paltry shooting percentage of 24.5%. The impact of 3-pointers is small, leaving the value added by the assist of 1.47 points. Note that this is the absolute upper bound on this value, assuming that every assist is a great pass that leads to an easy basket.
But this isn’t true, of course. Many assists come off of routine plays. If I’m at the top of the key and swing the ball to the wing, who then hits a jumper, chances are I’ll be awarded an assist. But I really didn’t do anything. I was just lucky enough to pass the ball to a guy who then made a shot. So a lot of assists don’t really provide much value at all.
So let’s take a step back here and think about what a good pass, the sort that might lead to an assist, actually does. A good pass finds an open player and gives him the ball in a position to score. To put it another way, a good pass creates a shot opportunity for a player, with a higher percentage than they could have gotten on their own. A good pass increases the FG% of the recipient. At an extreme case, it finds a player under the rim for a lay-up, resulting in an essentially 100% score, as in the analysis above. At the low end, it doesn’t do anything, and you just happen to get lucky and pick up an assist off of another player good play.
But basically, what you’d expect to see is some sort of correlation between assists and shooting percentage. More assists means more good passes for easier, higher percentage shots, and a concomitant increase in the FG%. If you can look at the numbers to find out what that relationship is—how much the FG% increases with increasing assists, you can then quantify exactly how much value the average assist provides to the team. And that’s what I’m going to try and do in part II of this piece, coming soon.