Player's Efficiency Margin, Explained

Here's the formula, with an explanation of what it is and how it works.

What it is:

This represents how many points a player is worth (how much they boost the point margin by) when they are on the floor for one possession per team (i.e. -- your team gets the ball, and your opponents' team get's the ball).

How it works:

This is the representation of two measures: the first is how many points a player scores, minus how many points their man scores (The NetTangibles value). This is an obvious good measure for how good a player might be overall (if they can score 20 points, they aren't worth much if they let their man score 50 every game).

The second is the more complicated one: it estimates how much they help their teammates (their intangibles). We take the expected help based on how many points their teammates and man's teammates scored (times the % of minutes the player played) combined with their NetTangibles, and subtract this from how much help the player actually gave (their PlusMinus value). This helps to accurately record how much a player's presence on the court actually benefitted their teammates above when they were off the court. This value will be higher for good point guards, worse for players that turn the ball over a lot, etcetera.

Then we simply estimate how many possessions the player was on the court for (a fairly accurate measure, Google 'estimating possessions played') and divide.

Edit: BUT ALAS, there is one problem with this formula. As a players' minute% reach 100%, the only value that comes into question is their Net Points (that is to say, their intangibles become zero). So our BEST GUESS for how much a player helps a team that also plays every minute of the game is simply their Actual Plus/Minus (PMa). So our best guess is a weight between the prior formula and their amount of minutes played: