Do Blocks Help?

Get that weak stuff outta here! Of course blocks help. Why wouldn’t they? There’s no better feeling as a big man than swatting your opponent’s naive attempt at a basket into the third row. Nowadays, however, with more data, there’s a bit deeper to dig when it comes to correctly valuing the NBA blocked shot.

The first question I wanted to answer in my study was whether players who get blocks were better on defense. Using data from Basketball Reference this season, here’s how a player’s defensive win shares per minute correlates with blocks per 100 possessions:

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There’s clearly a relationship there, but it doesn’t seem as strong as one might think. Perhaps blocks are far more important for big men:

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When running the plot for only power forwards and centers, the results are similar: blocks do seem to correlate with defensive win shares, but they don’t look as important as we think – especially when perhaps the most-used indicator of a big man’s defense by fans and broadcasters alike seems to be how many blocks they get per game.

My next question was whether teams that get more blocks fare better on defense. To attack it, I ran a regression model in the statistical programming language R with data for this season from Basketball Reference to find out the relationship between defensive rating (an estimate of points allowed per 100 possessions) and the three main defensive box score stats, adjusted on a per-100 possessions basis: defensive rebounds, steals, and blocks. After removing outliers and players with small sample sizes, the summary of the model is as follows:

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For those not familiar with R, the part of the output that’s most notable here is the number of asterisks in the rightward red box, which represent an estimation of the significance of that statistic in predicting the response – in this case, defensive rating. As you can see, while defensive rebounds and steals were important, blocks were considered statistically insignificant. Here’s a plot of team defensive rating against blocks for a better visualization:

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Clearly, the number of blocks a team amasses has had no visible correlation with points allowed per possession. Combined with the less stark insight that we found in the player data, blocks seem to be far less important than the other two defensive box score stats. But why?

Let’s keep it simple – at the end of the day, the goal of any defensive possession is to return the ball back to your team without giving up points to the opposition. Clearly, defensive rebounds and steals accomplish just that – both events result directly in a possession change. Blocks, on the other hand, are more complicated. When a shot is blocked, it may result in a defensive rebound, but ofttimes the offense grabs a hold of the swatted ball, or it sails out of bounds. As a matter of fact, data analysis from Nylon Calculus shows that a defense is less likely to regain possession after a block than after a normal miss close to the basket:

block outcomes 1415

Therein lies the issue – not all blocks are created equal. When it comes to getting the ball back to your team after a block, players have varying success rates. Again courtesy of Nylon Calculus, here were the top five players in terms of defensive recovery post-block last season:

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Amir Johnson was the best, and it’s one of the reasons he’s such an underrated defensive player. It’s held true this season, as according to ESPN’s defensive real plus-minus, he’s the 8th best defensive power forward in the NBA. He ranks 18th in the NBA in individual defensive rating, and 8th in the NBA in defensive box plus-minus.

Dwight Howard isn’t on that top five list – the player becoming more famous for his poor leadership and weak mental game than his skill clearly isn’t doing himself any favors with his blocks, as a league-high 35.8% of them were sent out of bounds last season:

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Of course, sending a shot out of bounds probably will yield the biggest “OH!” from the crowd and benches, but it functions effectively as a reset button for the offensive possession, far less effective than a tap in the direction of a defensive teammate. Less than half of Howard’s blocks last season (45.3%) resulted directly in a Houston possession. Reflecting this are his career-worst defensive rating of 105 (Amir Johnson stands at 100.8) and defensive box plus-minus of 1.3 (Johnson’s: 3.3).

Statistics really exist in sports for one reason – to measure the performance of players and teams. Clearly, though, the block is a far less useful measuring stick than many others, considering its poor predictive power for defensive performance and its widely varying effectiveness by player. A straight-up jump that contests the shot or a volleyball tap to a teammate may not be as sexy as an all-out rejection, but they are both more effective.

But hey, just for kicks, look at how disrespectful this was!


by Derek Reifer, Northwestern University


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