How are ratings for tactics puzzles calculated?

Well a puzzles rating then doesn't represent the skill that requires to solve it if only 50% of such players will solve it.

@blackzombie

It doesn't work like that.

1) ratings are a statistical definition. Being 2400 doesn't mean exactly being 2400 but rather somewhere in a distribution around 2400, hence it's normal for players not to always win against a slightly inferior opponent/puzzle

2) all players have their good and bad moments, so it's normal for players with the same rating to perform differently

3) in puzzles time plays a key role. A 2400 training quickly will make mistakes more than his rating should let him do. But if he took more time he'd be able to solve more puzzles

4) the rating is accurate indeed. Having the same rating as an opponent means, on average, not having any edge over them. So, obviously, a player with the same rating of the opponent is equally likely to win or lose.

Well! When it comes to puzzle I always thought one puzzles rating represented the level of skill typical for someone that manage to solve it without problem but I was wrong maybe.

Maybe Thibault could chime in and let us know how this works. It is probably similar to when players play each other, but trading puzzle points instead of rating points. I remember just starting out on the puzzles where my puzzle rating had huge changes (like +230) on the first puzzle, then is settled down and now I mostly see changes of +/- 15 or so max.