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The effect of a blocked jump shot on future jump shots in the NBA

April 30, 2013 - , , (1 Comment)

If an NBA player gets a jump shot blocked, does it change the way he plays the rest of the game? You can imagine there could be a psychological effect like a loss of confidence, or a conscious/subconscious decision to try harder to avoid being blocked again which could harm shooting efficiency. Basketball statistics legend Dean Oliver recently Tweeted the claim that it has a big effect on Steph Curry and basketball players in general. But is it actually true? And how big is the effect? Let’s look at some data.

Here’s the plan:

  1. For each player, find every game in which they had jump shot is blocked
  2. Calculate their field goal percentage (FG%) before and after their first blocked jump shot of the game
  3. See what the data tells us

Doing this requires a lot of play-by-play game data that allows me to look specifically at jump shots (no dunks). One source of this data I found was BasketballValue.com. They have data from 2005-2012; however, the first season is in a different format so I am ignoring it. So I have 6 seasons of data, from the 2006-2007 season up through last year. They do not (yet?) have data for this year, so all of the analysis below does not include the 2012-2013 season.

So I did the analysis as I described above (data and source code are on GitHub). First, I looked at the entire league, pooling together all players. The average FG% for a jump shot from 2006-2012 was 36%. The average FG% for a player who did not have any jump shots blocked in a game was also 36%. But what about players who did have a jump shot blocked?

  • Before blocked jump shot: 38%
  • After blocked jump shot: 34%

This is no statistical fluke. These numbers are from tens of thousands of shots by thousands of players in thousands of games. The sample size is large.

However, for individual players, the sample size is small. Steph Curry has only taken 289 jump shots in games where he had a jump shot blocked. And from those shots, he actually made a higher percentage (34% vs 29%) after he had a jump shot blocked. This goes against the leaguewide trend, but it’s not horribly rare. Below you can see a table and scatter plot for every player who has taken more than a handful of shots in these situations.

I would not read much into the value for individual players, since the sample sizes are relatively small, but the leaguewide trend is clear. NBA players shoot worse after they have a jump shot blocked.

Why? Maybe they are overcompensating. Maybe they are scared. Maybe they play differently later in the game than earlier (due to the score, for instance), regardless of if a shot is blocked or not. Who knows? Certainly not me. If anyone wants to investigate further, you can just take my code and start hacking.

Before Block After Block Overall
Name FG% difference FG% FGA FG% FGA FG% FGA

1 comment »

  1. You need to control for lineups. It may be that the decline is partly due to a rim defender entering the game and being successful, and thus staying in the game. That would lower the “after the block” fg%, but would not be due to a loss of confidence, but rather due to actual defensive improvement in the opponent.

    Comment by floppymoose — May 2, 2013 @ 6:33 pm

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