Friday, April 11, 2014

Regular Season vs. Playoffs: Contest+ %

Do teams contest more shots in the playoffs?  I think we'd expect the answer to be a resounding "yes" since, intuitively, contesting shots seems at least in part related to effort, and don't players try a little harder in the playoffs?  But the results are not so clear.

First, let's refresh on the definition of Contest+. We define Contest+ as any shot that is altered, blocked, or contested. Contest+ % will simply be the percentage of shots that are altered, blocked, or contested. For an exact definition on those terms, review this article.

Now, let's explore the methodology and results. I looked at Team Contest+ % for a sample of regular season games and the playoff games in the 2011-12 season. For those interested in just seeing the results and the conclusions, skip ahead. First, I conducted Levene's test for equality of variance and found that we have to reject the hypothesis of equality of variances in the two samples (p-value < .01). Next, I conducted Welch's t-test to see if the means were statistically different. Here are the results:

t-Test: Two-Sample Assuming Unequal Variances
Reg Season Playoffs 
Mean 0.4729023 0.456445
Variance 0.0080041 0.004745
Standard Deviation 0.0894656 0.068886
Observations 905 168
Hypothesized Mean Difference 0
df 283
t Stat 2.7022436
P(T<=t) one-tail 0.0036517
t Critical one-tail 1.6502557
P(T<=t) two-tail 0.0073034
t Critical two-tail 1.9683819
*Note: Playoff teams had a regular season Contest+% of 47.5%

Notice that Contest+ % in the playoffs has a lower average than the regular season. Moreover, not only is the average lower, but the difference in means is statistically significant (p-value < 0.01). So what do we make of this? Does this actually mean that teams contest a lower % of their shots in the playoffs than in the regular season? The evidence certainly seems to say "yes." However, before coming to this conclusion, I think we have to keep a few things in mind. First, the sample sizes are drastically different and there are nowhere near as many playoff games than regular season games. Second, there are fewer teams in the playoffs, and theoretically better teams that may be just better at getting open looks.  More than a quarter of the playoff games are coming from the Heat and the Celtics, who played in 23 and 20 games, respectively, that year. Finally, the actual difference in averages is not that large (just 1.6%). Regardless of whether it is statistically significant, this isn't a major difference. So with all this being said, while we may not be ready to conclude that players contest fewer shots in the playoffs, one thing we can safely say is that they aren't contesting more shots. And that is surprising. 

So what about our expectation that teams give more effort in the playoffs and, therefore, should contest more shots?  The fact that players don't contest more shots in the playoffs (despite what we assume to be marginally better effort) suggests one of three things (or some combination): (1) teams can't shake bad habits in the playoffs that were developed in the regular season; (2) skill is the determining factor in contesting shots, not effort; or (3) offenses that are good at getting uncontested shots trump the marginal increase in contesting shots that greater defensive effort produces. 

I'll end this post with a graph of Contest+ % for each playoff game for the conference finalists:


As you may be able to tell from the graph, the Celtics had the smallest variance while the Spurs and Heat had the largest variances. The Thunder also had three of their four worst games in terms of contesting shots in the last three games of the Finals, and this general downward trend is consistent across the teams to a lesser degree.

In part 2 of this article, we'll explore Open+ Frequency, how it changes from the regular season to the playoffs, and its relationship with Contest+%.

If you have any questions, comments or suggestions, please reach me on twitter @knarsu3 or e-mail me at krishnanarsu3@gmail.com 

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