Possessions are the foundation of a team's offensive and defensive efficiency.
A team can't score if it doesn't have the ball.
Basketball has evolved over time all because of possessions.
Dean Smith ran the 4 Corner Offense to stall the game. In the 1982 ACC Championship game, North Carolina held the ball for almost 7 minutes in the second half and beat Virginia 47-45.
By 1985, the NCAA adopted a shot clock to end this strategy. And in 2015-2016 season, the NCAA reduced the shot clock to 30 seconds.
These rule changes increase the amount of possessions per game.
Possessions is not an official NCAA statistic. It must be estimated.
In a perfect world, possessions would be defined by an action on a box score. A possession ends in any of the following situations:
A shot is made
A shot is missed and rebounded by the defense or offense
A free throw is made
A free throw is missed and rebounded by the defense or offense
The offense turns the ball over
The game or half ends
This isn't always what happens or how a team gains a possession.
In large part because of fouls and free throws.
For example, teams are awarded 1-and-1 free throws after their opponent commits 7 team fouls. If the first free throw is made, a second one is awarded.
If a player is fouled shooting a 2-point shot, it gets 2 free throws. A player can also be fouled when shooting a 3-point shot, meaning 3 free throws are awarded.
A player can be fouled on a made shot, and earn an extra free throw. Known as an and-1.
These variables mean possessions must be estimated because you can't always know how a team gets to the foul line.
Possessions have 4 main ingredients:
Field goal attempts (amount of total shots a team takes)
Amount of offensive rebounds
Free throw attempts
Possessions = FGA - OR + TO + (.475 * FTA)
The free throw multiplier is required because it's difficult to determine how a team made it to the foul line.
A team awarded 2 foul shots accounts for 1 possession. If it a team is awarded a foul shot after a made basket or gets an and-1, the single foul shot doesn't result in a new possession. The made shot is the only possession.
KenPom uses the .475 multiplier. The .475 multiplier estimates how many free throw attempts equal 1 possession. The key here is it's slightly less than one-half. This figure comes from play-by-play data.
Others have used .44, .40, or .42 as the multiplier in the past. The difference between these and .475 comes to about a 1% change in the efficiency calculations.
Possessions are counted for both teams and averaged.
Let's take at the 2018 NCAA Tournament game between Marshall and Wichita State.
Marshall attempted 60 shots, recorded 7 offensive rebounds, 9 turnovers, and attempted 22 free throws. This results in 72.45 possessions.
Possessions = 60 - 7 + 9 + (.475 * 22) = 72.45
Wichita State attempted 65 shots, grabbed 16 offensive rebounds, turned the ball over 15 times, and attempted 17 free throws. That's 78.30 possessions.
Possessions = 65 - 16 + 15 + (.475 * 17) = 72.08
The average of these two numbers is 78.27.
The game had around 72 total possessions (Marshall won 81-75).
This is the foundation that is used to manage how efficient each team was in the game.