Why the Sixers are in better shape than you think after Game 1

During Monday's 117-101 Game 1 win over the Philadelphia 76ers, the Boston Celtics looked nothing like an underdog in the series. Behind a combined 83 points from Al Horford, Terry Rozier and Jayson Tatum, the Celtics handed the Sixers their worst loss in almost two months.

Boston couldn't miss from beyond the arc -- the Celtics went 17-of-35 from 3-point range, including seven from Rozier in nine attempts and two from center Aron Baynes, half of his career total during the regular season (four) -- and Philadelphia couldn't hit. The 76ers shot 19.2 percent on 3s, their worst accuracy since January.

Yet Second Spectrum's advanced measure of shot quality told a wildly different story about Game 1. Despite Boston getting more 3-point attempts, Philadelphia's shots rated better in terms of expected value. Is that reason to believe the Sixers' chances of winning the series are better than they currently look?

Shot quality more sustainable than shot-making

Two years ago, during the 2016 playoffs, I used Second Spectrum's shot-quality data to back up the cliché that the NBA is a "make or miss league." Indeed, a team's shot quality -- as measured here by quantified shot probability (qSP), which estimates the effective field goal percentage (eFG) we'd expect based on where a shot was taken, its type, nearby defenders and the ability of the shooter -- proved several times less important in determining who won or lost a game than quantified shot-making (qSM), the difference between a team's actual effective field goal percentage and its qSP.

I also found at the time that shot quality tends to be more sustainable from game to game than shot-making. So if you're looking to predict how well a team will shoot in the following game, as measured by eFG, you're better off considering its shot quality in the previous game than its shot-making.

It turns out, with four-plus years of tracking data for the playoffs, we can now say the same is true for the remainder of the series. Compare how much stronger graphically the relationship is between a team's shot quality in Game 1 of a playoff series and its eFG in all the remaining games combined, than the same graph with its actual eFG in Game 1 instead of shot quality.

The correlation between a team's qSP in Game 1 and its effective field goal percentage the rest of the series is twice as strong as the relationship with its eFG in Game 1. In fact, if you're looking to predict how well a team will shoot the rest of the series, you're best off looking only at its shot quality in Game 1 and ignoring how well it made shots altogether.

Shot quality wins series

Now, let's take the next step and consider what shot-quality data from Game 1 can tell us about a team's chances of ultimately winning the series. I broke down the final result of the 68 playoff series since 2014 into four categories based on the outcome of Game 1:

  • Home team win with better shot quality than its opponent: 38-2 (.950)

  • Home team win with weaker shot quality: 3-3 (.500)

  • Home team loss with better shot quality: 12-3 (.800)

  • Home team loss with weaker shot quality: 5-2 (.714)

This breakdown is fascinating. When the higher-seeded team wins Game 1 and has the better shot quality in the game, as measured by qSP, it's almost unbeatable in the series. One of the two exceptions to this rule -- Portland's upset of the LA Clippers in the 2016 first round -- was a series changed dramatically by injuries to Blake Griffin and Chris Paul during Game 4. (The other, intriguingly: the Cleveland Cavaliers' comeback from a 3-1 deficit to beat the Golden State Warriors in the 2016 NBA Finals.)

Meanwhile, even losing Game 1 at home hasn't been a big obstacle for teams that lost because of shot-making. Those teams have still gone on to win far more often than teams that win Game 1 at home yet have worse shot quality. In other words, over the past four-plus years, you'd be better off losing Game 1 with better shot quality than your opponent than winning it with worse shot quality.

Now, the small sample size of the latter group is an issue. There's no reason to believe teams would also be better off losing Game 1 than winning given worse shot quality, despite the fact that the losers have in practice gone on to win a slightly higher percentage of the time. But overall, knowing the shot quality of the two teams in Game 1 will give you a better idea of who will ultimately win the series (the team with better shot quality has won 81 percent of the time) than knowing who won Game 1 (has gone on to win 68 percent of the time).

That's good news for Philadelphia.

Sixers got better shots in Game 1

We should pause here to note that Second Spectrum offers two measures of shot quality. As compared to the one we have been using, qSM, quantified shot quality (qSQ) differs by estimating the results if an average player took the same shot attempts. By qSQ, the Celtics had the better shot quality in Game 1 -- partially a function of Boston taking 35 3-pointers to Philadelphia's 26.

To some extent, however, that was by design. The Sixers should feel relatively comfortable letting Marcus Smart (who went 2-of-9) shoot 3-pointers, whereas Philadelphia's attempts came primarily from high-percentage shooters with the notable exception of center Joel Embiid. So factoring in the shooter using qSM shows that the 76ers should have been expected to post an eFG of 52.2 percent on their shots in Game 1, as compared to 50.4 percent for the Celtics.

In reality, both teams' shooting diverged wildly from what would be expected based on Second Spectrum tracking. Boston actually shot an effective 58.2 percent from the field, while Philadelphia sunk to 45.2 percent.

Relying solely on shooting evening out isn't necessarily a great strategy. After all, lower-seeded teams have won the series only half the time in the past when they've had better shot quality in Game 1 but lost the game. (One of those cases where the higher seed went on to win anyway: the Celtics' first-round series against the Milwaukee Bucks.) So it's good that Sixers coach Brett Brown is looking beyond the shooting in Game 1 while acknowledging its importance.

"I think to strip it down and just say they made 3s, we didn't, I'm not prepared to go there entirely," Brown told reporters between games. "I think there is some truth to that. ... I think the 3s by and large that we took, you'd take most of them again."

If Philadelphia keeps taking the same shots, we'd expect more of them to go in. As a result, the 76ers are in better shape in this series than they appeared on Monday.