Sunday, April 27, 2014

Win Probability Added and the Four Factors

Having a terrifying mascot is not
one of the four factors
I have a new feature up on my win probability box scores. For each game, I breakdown team win probability according to the "four factors" of basketball success. The four factors are: field goal shooting, getting free throw attempts (and converting them), rebounding the ball, and turnovers. The four factors were first identified (and named) by Dean Oliver, pioneer of basketball statistical analysis.

Most everything that is important to winning and losing basketball games can be attributed to one of the four factors. Similarly, nearly every shift in win probability that takes place in a game can be assigned to field goal shooting (FG), free throws (FT), rebounds (RB), or turnovers (TO). What now emerges in the box score is a unique fingerprint for each game; an accounting of how each phase of the game contributed to the final outcome.

Because a winning team starts with 0.5 win probability and ends with 1.0, the difference between the victor's win probability added for the four factors combined should be 0.50 more than the losing team's total. It rarely works out exactly though due to plays that are not attributable to one of the four factors (e.g. end of quarter plays that don't result in a field goal attempt). But for most games, it is fairly close to a 0.5 difference.

For most games, the field goal WPA tends to be the difference maker, in line with the general consensus that field goal shooting is the most important of the four factors. Take game 4 of the Hawks-Pacers series. The Pacers held a 0.77 WPA advantage in field goal shooting, enough on its own to secure victory. The Hawks however, had a 0.44 edge in free throw shooting. Rebounding was a draw, but the Hawks lost the turnover battle by a margin of -0.28 in WPA.

And check out the FG WPA totals for game 3 of Spurs-Mavs. A game that was won and lost several times over on the floor.

No comments:

Post a Comment