You might wonder why I used different rating scales for importance and satisfaction. Part of the reason is that there are two types of rating scales: unipolar and bipolar. A bipolar scale goes from negative to positive, whereas a unipolar scale goes from 0 to 100 percent of an attribute. It's usually best to measure satisfaction using a bipolar scale; since people can be satisfied or dissatisfied, a negative score makes sense. In contrast, importance is just a matter of degree—without any negative value—and therefore better measured with a unipolar scale.
