The concept of total points added (TPA), isn’t particularly complicated. We’re looking at both defensive and offensive effectiveness on a per-possession basis while also incorporating the amount of playing time the contributor in question receives.

At the heart of the theory is this comparison between two hypothetical players:

• Player A makes an average team 5 points better per 100 possessions than an average player would in his spot, and he plays 500 possessions.
• Player B makes an average team 10 points better per 100 possessions than an average player would in his spot, and he plays 250 possessions.

Player B is more effective on a per-possession basis—twice as effective, in fact. But Player A spends twice as much time on the court. Theoretically, they should have identical values, as they would both add 25 points to an average team.

As such, the formula for TPA is rather simple. It’s broken down into two parts—offensive points added (OPA) and defensive points saved (DPS)—and each is calculated in the same vein.

OPA is derived by adjusting offensive box plus/minus (OBPM) to account for the number of possessions the player in question is present for. Similarly, DPS is derived from a similar adjustment of defensive box plus/minus (DBPM) with that same number of possessions. OBPM and DBPM, both calculated by Basketball-Reference.com, estimate the per-100-possessions value of a player on either end of the court.

Add OPA and DPS together, and you have TPA. A score of zero indicates a player was perfectly average (by no means a bad thing for rookies or lifelong end-of-bench players), while anything positive means they were better than an average-level replacement.

For scores prior to 1972-73, we’re estimating OBPM and DBPM using historical correlations with offensive and defensive win shares per 48 minutes. That data goes all the way back to 1951-52, allowing us to come up with rough estimates for the early legends of the sport. These numbers are by no means perfect, but they can still serve as baselines.