one of the things that keeps bugging me about this debate is just how fundamentally crazy it is that people will reject something like expected goals in favor of something like +/-.
+/- tells you literally one thing and one thing only: was this player on the ice when a goal happened at even strength? Goals are obviously important, it's like the whole point of hockey, but in a sport with 12 guys on the ice most of the time, just knowing if a goal was scored tells you next to nothing about whether that guy is any good or contributed to that goal in any meaningful way.
^ this is what we mean when we say +/- is a flawed stat. It's just extremely limited in its usefulness in evaluating individual players on its own. Even over a long period of time if a guy has a really positive +/- it doesn't tell you with much certainty if that player actually played well. Maybe he's just the 5th wheel on a really excellent team.
On the other side you have something like expected goals (xG). First, it's not contingent on whether an actual goal was scored, which means you have a lot (orders of magnitude) more data points to work with. This makes the data less prone to "luck". Additionally xG models include information about the type of shot, where was it taken, how dangerous it was, etc.
Is xG perfect? Does it tell you everything you could want to know about a scoring chance without seeing it? Obviously not. It doesn't tell you about the passing play before the shot, or the competence of the shooter, for example. It also doesn't tell you if they were scared of a big, tall defenseman nearby or if they had bubble-guts from the wings they had last night. But even just the basic shot quality & location information still tells you a hell of a lot more about the individual player's performance than +/- does.
So given all of the above, why would anyone ever reject the use of something like expected goals when it simply gives you more, higher fidelity information than another stat like +/-. And this is especially baffling to me when you are allowed to use both! this doesn't have to be a choice. It's not religion, people! It's just information!
I'm going to go out on a limb here and open myself up to ridicule and explain why I struggle with expected goals for, and a lot of the other "projections", while at the same time fully stating a willingness to be objective about them and wanting to learn more.
The problem I (and I'm guessing many people have) is this ----
Hockey is so free flowing an unpredictable that literally a stick blade being 8 inches to the left or right or a shot hitting the bottom portion of a 4 inch cross bar, as opposed to the top portion provides entirely different results.
Every single player could do everything right during a shift and McDavid can snipe a top shelf goal from an impossible angle and that somehow leads to a prediction regarding the players that were on the ice at the time and what will happen in the future, even when players less skilled than McDavid are the opponent.
To me, what McDavid does when a certain line is on the ice is going to be different when Draisaitl is with him....which will be different when the two of them are paired with Pulijarvvi vs. when they're paired with Hyman.
Sure, each of them have tendencies, but do they have tendencies as a line? (I'm sincerely asking -- do they track these stats based on who they are playing with and can those stats be pulled up immediately during a game?)
For instance, I think it stands to reason that the Bergeron - Marchand pair plays differently when Pasta is with them vs. JDB.
Do the stats show that Marchand shoots less with Pasta than JDB? Does Bergeron do something different on the faceoff when JDB is on the ice with him?
Projecting or predicting what is going to happen in a given game or season just seems to be incredibly tenuous simply because of the speed of the game and immense variables that exist.
I get that the predictive stats don't exist to guarantee anything, but they also don't/can't take certain things into consideration such as a players health, effort, ability to perform under pressure etc.
If I can jump to baseball for a second. I'm sure that there are/were predictive models that we could have looked at to determine the likelihood of David Ortiz getting a hit or making an out in a playoff game vs. a particular pitch or pitcher based on what happened throughout that season, his matchups with that pitcher or his career in general.
I don't think there's a person alive who watched that guy play here for 15 years or so that would put one iota of credibility in anything that suggested he'd make an out with the game on the line.
And, IMO baseball has fewer variables than hockey.
Lastly, I'm not trying to be argumentative -- just being honest about the things that I think about with predictive stats and I'm happy to learn more about them.