Playoffs Pessimism Part II: Are Stretch Runs Sticky?

Photo by Melissa Majchrzak/NBAE via Getty Images

In my previous post, I evaluated a claim that optimistic Jazz fans make: That a team that improved significantly during last season will get even better this year. I found, however, that such back-to-back jumps in wins are rare — alarmingly so.

However, as many fans point out, the Jazz don't really need to improve. All they need to do is to play as well as they did after last year's All-Star Break. Those Jazzmen, of course, went 19-10 down the stretch, significantly better than their prior record of 19-34.

What most Jazz fans think — or at least hope — is that the team's late season run is more predictive of how they will be do this coming season.

That, my friends, is a proposition we can test!

Let's turn to the data and ask a simple question: Are the 2015-16 Utah Jazz likely to win as much as they did after the All-Star Break (their winning percentage of .655 translates to a 54-28 record)? Or will their mark be closer to their full season total of 38 wins?

In other words, do stretch runs stick?

The Data

Just as with my previous post, I looked at the last 11 NBA seasons, back to 2004-05. I looked for teams that either improved or declined significantly after the All Star Break.

The research project was simple (but time consuming!): Find all the teams with big late season jumps or declines, then see how those teams did the following year.

A reasonable cut off for a “big” post All Star Break change was a change in winning percentage of .200. The Jazz easily fit in: They improved from .358 team to a .655 team, a difference of .297.

How many teams improved by at least .200? In the past 11 years, 17 teams have. Of those (and we'll list them at the bottom of the post) the Jazz actually had the fourth-biggest improvement behind only the 2004-05 Nuggets, the 2004-05 Warriors and the 2006-07 Sixers.

Here's another way to look at how much the team improved: The Jazz jump during the 2014-15 season was the equivalent of going from the West's 13th-place team to its seventh-best. Before the All Star Break, they played like the Kings. After, they were nearly as good as the Spurs.

That's improvement.

Now, if we're going to study how much changes “stick,” it's worth our time to also look at those teams who get much worse after the All Star Break. So how many teams have gotten at least .200 worse after the break? 19. Only one, the 2006-07 Pacers, saw their record plummet by as much as the Jazz skyrocketed last year.

And More Data!

OK, so how do those big-change teams fare the following season?

Let's start with the improving clubs. We now only have 14 teams to work with over a 10 year period. (The other three — the 2014-15 Jazz, Celtics and Pacers — all had their jumps last year. Their fates are undecided!)

Those 14 teams, post All-Star Break, had an average record of 22-8, for a winning percentage of .735., which was .261 higher than their pre-break marks of .475. And the next season?

Those teams averaged 40 wins and 42 losses, for a winning percentage of .494.

Hmmmm. In the year after their big post All Star jumps, those teams won at a rate .242 lower than their late season hot streaks. In other words, those teams in that following year gave back, on average, 93 percent of their gains.

Whoa. Let's look that data visually:

How “Big Jump” Teams Fare the Next Season

Now, let's look at the declining teams. Here we have 18 to study (the 2014-15 season saw just one team whose record plummeted, the Raptors.) Those 18 teams, post All Star Break, had an average record of 10-20, for a winning percentage of .345, which was .236 worse than their pre- break marks of .581. And the next season?

Those teams averaged 42 wins and 40 losses, for a winning percentage of .513.

So, during this decade of study, late-season collapses were a bit “stickier” — but still not that much: Their winning percentages in the year after the big post All-Star decline were .168 better than during their late season collapses. The decliners thus gained back about two thirds of their success the following season.

The Discussion

I went into this study feeling fairly confident that late season runs (and collapses) would turn out to be not as predictive as most fans might think. I'll admit I'm genuinely shocked how little those final 30 games tell us about how a team will do the next year — especially when a team has a hot stretch run.

It turns out the best predictor for how a team will do next season? By far, it's how they did for the entire season, not how they did down the stretch — and it's not even close. Let me share a bit of fancier math and run some simple statistical analyses.

“Correlation” is a statistical concept which tells us how much one set of data is linked to another. In this case, it's a simple tool for observely how closely a following year's winning percentage is tied to previous records.

I took the data for my 32 “big-change” teams and ran correlations to see how well their winning percentages in the subsequent sedason correlated with three variables: The team's record post-All-Star Break, pre-All-Star Break and for the whole season. The result ranges from 0 to 1. 1 would mean the two sets of data march in lockstep with one another; 0 means there is no connection.

A correlation of .3 is considered a “weak” relationship, .5 is considered “moderate” and anything above .7 “strong.”

Pretty clear, no? Most surprising to me is not only do full season records correlate better with the following season's record than the post-All-Star marks, but the strongest correlation is actually the pre-All-Star record.

Not only are these teams worse than they were during their hot streaks, they're actually a bit closer to what they were at their lowest.

Why? Clearly, there is no one answer. But let's speculate about a few likely causes. During their “hot streaks,” teams enjoy excellent health, which often doesn't persist the next year. Other teams notice that hot streak, begin to take that team more seriously and scout and plan for them better. Some of the “hot streak” is just dumb luck: Shots fall that typically wouldn't. The teams win a disproportionate number of close games. A few calls go their way. They run into tired teams, or teams resting a key player, or teams battling injuries. The next year, those trends tend to return to the average.

Did the 19-10 Jazz benefit from a few of those trends? Probably.

Now, let's state the obvious: Data is not destiny. And, as they say in fine print on those ads for financial products, Past Performance is Not Necessarily Indicative of Future Results. Just because late season hot streaks haven't “stuck” for nearly all teams, that doesn't mean it won't for the 2015-16 Jazz.

However, just as with the last post, optimistic Jazz fans anticipating smooth sailing to a playoff spot need to realize that history — and math — aren't on your side.

One final caveat: Some fans have suggested in comments that the Jazzmen are more immune to these trends because of the nature of the team's big jump, largely due to a massive improvement in team defense — and the insertion of one long-armed French center into the starting lineup. Defense, the argument goes, is less fluky. Once you make a big jump on D, you stay there, assuming coaching and personnel broadly stay the same.

In other words, maybe the Jazz are better poised to buck these trends, as a D-first club. Folks, I sense another proposition to test!

Stay tuned.

P.S. Here's that full table for all you gluttons.

First, the late season hot teams: