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Conditions for evaluating

Example of calculating plus / minus, net points: A basketball team has one topper and one good center, the topper plays 40 minutes per game and is intrinsically rated + 6 net points per 40 minutes, the center is playing only 20 minutes per game and is intrinsically rated + 3 per 40 minutes, the rest of the team has a 0 rating average. The center is always playing with the topper (no separation) and both are never in garbage time. Because he is always playing with the best team, in this strictly theoretical model, the center is getting a 3 + 6 = + 9 net points.
The topper is playing halve of his time without the center’s help. This is the separation period for the topper. The topper looks less and is rated (9 + 6)/2 = + 7.5 net points.
Conclusion: In this simple context, it is misleading to evaluate and to extrapolate the performance without separation and it comes to nothing without using only good samples of time and equal game conditions, without structure.
If a lot of conditions are not met in the method of evaluating, it is not possible to evaluate (the performance of the basketball players with their complex skills) correctly .

Another problem with using net points to evaluate the performance of individual players is that it is biased when you play with better teammates or against weaker opponents, without adjustments. It is a little biased too with a player playing with the ideal chemistry of team mates, compared to a better player but playing in, for him, the wrong team (chemistry).
A player who can play more positions ends better, he is filling up more missing links. The necessity of the player’s type, the chemistry he brings to the group, will be his fitting factor.

For a good evaluation of the performance you have to look at the playing time in which the players do not share the court, because only then they are making the difference. To control and adjust this seems very complicated, but there is a way.

Conceptually all evaluated players should be given a chance to perform in the same kind of competitive full-court game conditions. The same quality and quantity they play.
The same chemistry. The same sample of time. No home-court or other advantage, no garbage time. A good mix of line-ups to create and to spread the effect of the different interactions. The same coaching influence and motivation. Line-ups should have enough minutes to produce realistic quality estimates. The strength or weight in this competition is structured for each player. You have to do the test with a balanced and homogeneous group of players.

Every test gives us a single number of the basketball player’s performance value. Every test also gives us our total game results, our individual offense, individual defense and all round evaluations, together with ball bound, offensive rebound, offensive and defensive skills etc.

Ball bound is the total amount of active possessions or the total of offensive + defensive rebounds + steals – turnovers and the + - violations every player shares within his team.

The team score, the team possessions and the team offensive rebounds are very objective and very easy basic stats.

When we evaluate the performance of a player, we boil it down to a single number by dividing offense by defense.
A player or a team can win with the score of 62–50, that results in a 1.24 evaluation. 
That is better than winning with 112-100, which results in a 1.12 evaluation. 
In a simple but correct way the difference in pace of the game is eliminated.

In an equation, any quantity can be subtracted from both sides. We can strip again 50 points and the result is 12-0. But we can not do this in an evaluation.

We can also boil it down to two other numbers in these structured games by extracting the number of points every player was generating (producing) or the number of points every player was allowing.

Offensive evaluation fraction: Evaluation of the individual scoring offense = team scored points / ( team possessions – team offensive rebounds) in the presented tests.

Defensive evaluation fraction: Evaluation of the individual scoring defense = team allowed points / (team allowed possessions – team allowed offensive rebounds) in the presented tests.

If the team (allowed) possessions and the team (allowed) offensive rebounds are not equal, then the evaluation of the performance of the player is the team offensive fraction divided by the team defensive fraction. If they are equal, than the evaluation of the performance of the player is team scored points/ team allowed points.

Single Number Evaluation of the Player
Team Scored Points
divided by
(Team Possessions – Team Offensive Rebounds)
divided by
Team Allowed Points
divided by
(Team Allowed Possessions – Team Allowed Offensive Rebounds)