|Question:||"Scales" on a questionnaire should have how many points?|
|Answer:||The way to think about this question is as follows. The more scale points you have, the more information you will get. Thus, if you ask people a yes/no question, they can't tell you if they feel indifferent to the question (that would require a 3 point scale). Since surveys (which I assume you are using this scale for) are for getting information, the more scale points the better.|
Having said this, you can easily ovewhelm people with a large number of scale points. So giving people a large number isn't good either. In fact, with a large number of points respondents tend to demonstrate "floor" and "ceiling" effects, which means they answer the extremes of scale. This tends to through your results off.
One other issue is the use of an odd or even number of points. While some people want respondents to use an even number (and thus forcing a choice), this is really inconsistent with human behavior. The reality is that we often are not sure or are indifferent, and this should be reflected in the scale. So, an odd number of points is best.
Having said all of this, we would recommend a 5 point scale. It eliminates most of the problems just mentioned.