Someone asked friend of the blog Neil Greenberg how reliable individual +/- is. I think it's very reliable - in fact, you would do reasonably well signing the player who finishes dead last in the league in +/- every season provided he wasn't on an expansion team. Let's take a look at that list:
|Season||Player||GP||+/-||GP Year2||+/- Year 2|
||73||-46||no season||no season|
More than half of these guys were in the middle of otherwise fine NHL careers and found themselves getting a lot of tough ice time on an awful team and, on top of that, had all of the bounces go against them. The top half of these players went from an average of 74 GP and -38 in their bad +/- season to 73 games and a -7 in their next season.
This relates back to our discussion of PDO on Friday. A lot of bad +/- is driven by a low on-ice shooting percentage and a low on-ice save percentage - PDO. And PDO has zero year-to-year correlation, so it's difficult to infer much skill from a bad +/-. And clearly a lot of decision-makers in the NHL do get that (otherwise they'd cut their +/- trailers) but I'm willing to bet that guys with poor +/- are still undervalued.