Nylon Calculus: How would reducing the shot clock affect offenses?

Last summer, the NBA introduced a new rule reducing the time on the shot-clock after an offensive rebound from 24 to 14 seconds. The reasoning for the rule change was to increase the pace, and indeed pace was up this season — although, it is unlikely that the rule change accounts for a significant portion of the rise. So what if we went further? If increased pace is a goal for the NBA, there are plenty of easy ways to do so. What if we cut the shot-clock down from 24 to 18 seconds for all possessions?

As it stands, the average offensive possession for an NBA team takes about 14 seconds and roughly one-third of possessions take 18 seconds or longer. So the rule change is certain to quicken a team’s pace. Still, the most interesting aspect of the possible rule change is its effect on decision making.

In The Problem of Shot Selection, Brian Skinner examined the choice of when to take a shot, and when to pass it up in search of a better one. The choice is based on the number of possible shots left in a possession, constrained by the shot clock and turnover rates. The fewer possible shots left, the less likely it is a better shot is possible and more likely it is that taking that not-quite-ideal floater is the correct choice. Conversely, the more time left on the shot clock, the more boneheaded that twenty-foot post-up looks.

Shots taken early in the clock tend to be of higher value than those taken later in the clock. Part of the explanation for this is the decision to attempt a shot is made only when that attempt has a higher expected value than the possible subsequent shots available to the offense on that possession. So an attempt early in the clock exists only because it is more likely to a net a higher amount of points than the following possible shots. Second, plays designed to net high-value shots are run early in the clock, with the knowledge that if the offense were to wait until the end of the shot-clock they would be highly dependent on the play without any options if it were to fail.

Charting average points per-possessions for this season over the course of the shot-clock we see a steep drop in points-per-possession from 18 to 17 seconds left on the shot clock, likely reflecting a change in the offense from transition to half-court play. Around 12 seconds there is a slight uptick in points-per-possession, possibly representing teams entering into the end result of their designed plays. The second-largest drop in points happens from eight to seven seconds left, a decrease of 0.11 points-per-possession, and a gradual decline follows with each second thereafter.

Reducing the shot clock to 18 seconds would not mean offenses would face the same curve, just starting at eighteen seconds. First, we would expect some adaptation — as it stands, half-court offenses don’t begin until about seven seconds into the shot-clock, with only 18 seconds available every tick is precious and point-guards would undoubtedly rush up the court more frequently. Second, we may expect an offenses efficiency to hold a little longer into the shot-clock — in the same way, designed plays net a slight bump around 12 seconds, we would expect plays netting efficient shots to happen with less time on the shot-clock then they currently do. A steep drop thereafter is an inevitability though, that last second heave is no more likely to go in regardless of if the shot-clock started with 24 seconds or 18.

Given how relatively flat the curve is from 17 to eight seconds left, the marginal gains from having an additional possible shot may not be all that large. The ability pass up one shot is certainly much better than having time to only catch-and-shoot, and going from having two theoretical attempts to three may also be a significant boon, but as those numbers get larger the increase in additional value decreases. For that reason, along with some offensive adaptation, while an overall decrease in offensive efficiency is likely, the resulting decrease may not be all that large. Still, the league would be walking a fine line between pace and offense.