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RPM: Spurs should be favored

6/5/2014 - NBA

The Vegas oddsmakers have anointed the San Antonio Spurs as the narrow favorite in this year's NBA Finals. The latest line translates to about a 57 percent chance of another championship parade down the River Walk. ESPN Forecast -- our panel of experts -- calls it similarly, giving Tim Duncan and crew a 54.5 percent likelihood of reclaiming the NBA throne.

But our real plus-minus forecast model sees the Spurs as the prohibitive favorite to win this year's title, with better than 3-1 odds of beating the Heat in this Finals rematch.

Admittedly, it's a bold projection -- especially if it turns out Miami really can shift into a higher gear. But the RPM numbers back it up.

Spurs sport superior power rating

To forecast the outcome of a given NBA game or series, we can start with the power rating for each team, as derived from RPM. To find the power rating (net efficiency) that applies to the Spurs and Heat for the Finals, we just need a two-step commonsense process: A) figure out each team's likely rotation (that is, the number of minutes for each player); and B) calculate the team's RPM rating based on the previous RPM numbers and the expected rotation.

This series presents an interesting challenge when it comes to forecasting the rotation for each team. For one thing, some key players are banged up -- particularly the Spurs' Tony Parker, whose recent ankle sprain represents a huge wild card. It's impossible to know in advance exactly how much (if at all) such injuries will affect playing time and effectiveness.

Also, both coaches (Gregg Popovich and Erik Spoelstra) are known for their willingness to experiment with new, relatively untested lineups at any point, even in the postseason. On top of that, each team has taken unusual measures to provide rest for its players during the season, with the expectation that minutes will be extended for key players in the Finals.

Still, the best predictor of future behavior is past behavior, and these teams have played a combined 33 playoff games this year, giving us a rich data set.

That's why, for my analysis, I simply assume each player on each roster will play the same proportion of total team minutes he's already played throughout the first three rounds of the postseason.

The result is that the Spurs have an implied Finals power rating of 14.12, with the Heat at only 10.64, a significant difference.

Depth should defeat superstardom (this year)

LeBron James is the best player on the planet, with a staggeringly high, league-leading RPM value of plus-9.19. That's the number of points he contributes to the Heat per 100 possessions (both offensive and defensive possessions).

Yet the Spurs still come out with a decisive edge in our analysis. In fact, the RPM model says they are better than the Heat by about 3 points per game (3.5 points per 100 possessions).

Why? In a word: depth. The Spurs have a ridiculous number of really good players, sporting eight guys on the roster with an RPM rating of plus-2.0 or better. That's high enough to start sniffing All-Star territory. In stark contrast, Miami has a top-heavy talent distribution, forcing the Heat to hand out key playoff minutes to players with negative RPM values, which drags down their net efficiency.

At the end of the day, the Spurs' depth prevails, big time.

San Antonio also has the home-court advantage for the series, worth about 4 points per 100 possessions.

When we put it all together, the Spurs are huge favorites when playing at home, with an implied edge of about 7.5 points per 100 possessions (which amounts to more than six points per game), while the games at Miami are about a toss-up (slight edge of 0.5 points per 100 possessions to the Heat).

What are the odds?

In this year's Finals, San Antonio has an efficiency edge of roughly 7.5 points when playing at home. For those four games, about 25 percent of the time, random chance will give the Heat an advantage of more than 7.5 points. Therefore, the Spurs should win in about 75 percent of matchups in San Antonio.

Applying the same logic to games in Miami, we find that the Heat should win about 52 percent of the time.

From there, it's easy to calculate the probability of each possible outcome for the series. (It just takes a little applied combinatorial math.)

To spare you the gory details and cut to the chase, the Spurs have a 78.5 percent chance of winning this year's title.

The following chart gives a more precise probability breakdown for each possible series outcome:

Don't forget random chance

Of course, randomness does come into play in a short series. Just as we understand probability when it comes to weather forecasts, we can apply the same logic here.

With the Spurs at almost 80 percent to celebrate another title, does that mean the Heat are doomed? Not at all.

If the Heat win, does that mean the previous stats were wrong? Not at all.

Life is full of random events.

For instance, the Heat were favored (by the same analytic model) to win last year's Finals, and they did indeed win -- but they would have lost if not for two lucky bounces in Game 6 and some other breaks. As Bill Simmons put it, "If you played the last 28.2 seconds [of Game 6] 100 times, San Antonio would probably win 99 of them."

So, yes, the Spurs can be regarded as a strong favorite this year. But the Heat might be just a couple of lucky bounces away from another title.