How do you determine a plus or minus rating in hockey?

It's quite simple really. If a player is on the ice when his team scores a goal, he gets a plus. If he is on the ice when the opposition scores, he gets a minus. For example, if during a game , the player is on the ice when his team scores 3 goals , but is on the ice when the other team scores two, he is a plus one (+1). However, power play goals are not factored into the mix, although shorthanded goals are.