NBA Preseason: Predictive or Pointless?

With the NBA preseason coming to a close, it may be time to actually start watching. At least, that’s what those of us who paid little attention to the preseason match-ups may tell ourselves. Yeah, Kawhi Leonard and Paul George might have got us all excited when they joined the Clippers, but that was months ago, and the NBA regular season has so. Many. Games. But did we lazy fans actually miss anything by choosing to go see Joker or binge Netflix instead? Are we missing valuable insight into how our favorite teams will fare?

To answer that question, we turn, as we always do when faced with such a serious quandary, to the numbers.

Daniel Cowan ‘22 took on the task of answering: does a team’s preseason record have any predictive value on that team’s performance in the regular season?

He examined data from the 2005 NBA season through the 2019 season, looking for any possible trend by plotting the team’s regular season winning percentage against the same team’s preseason winning percentage from the same year. He used winning percentage rather than wins to remedy the fact that not all NBA teams play the same number of preseason games. 

The trendline he plotted for the dataset resulted in an R-squared value of 0.08; the R-squared value of a given dataset demonstrates how much effect the independent variable (preseason winning percentage) has on the dependent variable (regular season winning percentage). Because Daniel’s R-squared value is very low, his findings indicate that preseason winning percentage has very little predictive value for regular season winning percentage.

Daniel did discover a few outliers, however, which suggest that some years are more predictive than others. When he plotted the data year-by-year, he found that the R-squared value for 2017 was 0.32. On the other hand, the R-squared value for 2007 was 0.01. All this to say, if your team does poorly in preseason, do not despair. All is not lost yet.

But at what point is all lost? The question this time was: How predictive is the start of the season for an NBA team, in terms of their finish?

He returned to the data from 2005-2019, comparing each team’s winning percentage in the first x number of games to that team’s winning percentage by the end of the year. He conducted three trials with x =15, 20, and 25 respectively. He plotted regular season winning percentage against the winning percentage during the first x games, and included a color filter based on whether or not the team made the playoffs. For instance, below we see the graph for the first 15 weeks. To see the entire Tableau workbook, visit: https://tabsoft.co/2MHE10c

He discovered that a team’s winning percentage through the first x games is highly predictive of the winning percentage by the end of the year, with the predictive value growing stronger as x is increased (R-squared value for x=15 was 0.63, for x=20 it was 0.70 and for x=25 it was 0.74). 

Daniel noted that this finding is unsurprising, as the season progresses, the good teams will separate from the bad teams. But his chart allows fans to see how clear the separation is and to know that good teams generally win early and keep winning, while the opposite is true of bad ones. So if your team bombed in preseason, keep the faith and keep watching. If your team stinks during the first 15 or 20 games of the regular season, it may be time to stop wearing that jersey in public.

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