[obaxedbg] which half point

rate chess players by how often they turn a drawn position into a lost one, and by how often they turn a winning position into a drawn one.  (turning a winning position into a lost one counts as one of each.)  the best at avoiding bad moves of the first type might not be the best at avoiding the other type.

devilish detail of determining ground truth of a position.  maybe play out by an engine.

devilish detail of normalization: some positions are easier to make mistakes in, and some players probably (as a result of play style) encounter such positions more frequently.

skill difference might matter: a subtle (but still half-point) mistake against a beginner might not matter, because the beginner might not be able to exploit it, or might be expected to blunder it back later in the game.

having a high rate at either or both of these categories might not equate to bad results -- perhaps you have a playing style that, even though you make a lot of mistakes, you induce your opponents to make even more.

having a low rate of making mistakes might not equate to good results: only one mistake per game is enough to lose every game.

opposite measure: how often a player induces an opponent to make each of the half-point mistakes.  similar devilish details, including normalization.