Quoting: Bf3351
Are you stupid? When someone is a minus multiply times and by more than -10 it's not a coincidence. They are bad defensively.
+/- is SUPER usage dependant, for a few reasons.
1. It's essentially an on-ice stat with no controls for quality of competition (impactful for certain players), zone starts (rarely a major factor, but relevant. Not really gonna hurt Stammer though), score effects (relevant), or quality of teammates (very relevant.)
2. It's strength controls are dumb. PP goals for don't count as pluses but shorties against do, so PP time can only hurt your +/-. Similarly, PK time can only help it (shorties for count but PK goals against don't.) The big one though is that 6v5 GA (goals against with an empty net) count as normal, so players with significant 6v5 icetime get REALLY heavily hurt by it.
3. It's a count stat, not a rate or percentage. This means high icetime is required for significant swings either way. The worst players in the league are usually close to average by +/- because they don't see much icetime, while the bottom of the ladder is populated by guys like Edmonton-era Taylor Hall, star players on crap teams. The fact that such players tend to get lots of PP and EN icetime doesn't help matters.
The other big issue over a small sample is that it's a goal based metric, which makes it really vulnerable to wild swings as shooters/goalies run hot and cold. There's a reason one of the first and most prominent luck stats in hockey (PDO) is just on-ice shooting% + on-ice save%. Goals are super volatile over any sample smaller than 3-ish years for an individual player.
I'll look up some other numbers on Stammer's D this year and see how he stacks up to past years. It's entirely possible he hasn't been good, but PLEASE stop using +/-. 5v5 GF% (the percentage of goals scored that were scored by a player's team with that player on the ice at 5v5) does essentially the same job but cleans up some of the usage stuff. Regression models (like Evolving-hockey's RAPMs) are even better, with the added bonus of separating out offence and defence.
Use better stats!