For years I have told students what I call the Number One Rule of Sociology:
We make generalizations about groups which do not necessarily apply to each individual in the group.
Often people err in treating a true group generalization as if, to be true, it has to apply to each person in the group. Since this is almost never the case, they dismiss the possibility of generalizing about groups at all. Yet group generalizations are essential in dealing with a world of strangers.
Recently I thought up a shorter version of this rule, or perhaps it is the prerequisite to the Number One Rule. To be a sociologist, you need a firm grasp on the difference between Most and All.