Research Design and Statistical Consulting
George M. Diekhoff, Ph.D.

How Many Rating Scale Anchors are Needed?

This time let’s talk about the use of odd vs. even numbers of scale points in Likert-type rating scales. I just worked on another project in which the principal investigator had chosen to use an even number of scale points–1-4–in assessing opinions: 1 = strongly disagree; 2 = disagree; 3 = agree; and 4 = strongly agree. What’s wrong with that? Where does it leave the poor soul who truly has no opinion on the issue or who is completely “on the fence?” The explanation given for using a scale with an even number of scale points is that it “forces” people to take a stand. Is that a reasonable thing to do? Isn’t the purpose of the scale to find out what people think, not to force them to think one way or the other? If someone has no opinion on an issue or is really torn equally between the two alternatives, that’s what a researcher should find out, but the 4-point scale doesn’t allow that.

Opinions, and the rating scales that measure them, tend to be bipolar dimensions. That is, you can like the President’s position on an issue or not like that position. Two poles, one indicated by low ratings (1 or 2), the other indicated by high ratings (3 or 4). Sometimes, though, we use rating scales to measure unipolar dimensions, i.e., the degree to which something is present or is the case. For instance, I might ask you to rate the degree to which your first statistics class was a rewarding experience from 1 = it was a train wreck! to 5 = it was magical! In a case like this, the use of an odd number of rating scale points that provides for a center point is even more important. Think about how most attributes are distributed. Normally, right? The bell curve. That is, most cases score IN THE MIDDLE. Those in the middle ranges of an attribute need a place to mark their rating scale and it needs to me in the middle. Only if the scale is 5 points, 7 points, or maybe 9 points is there a middle position to mark. With an even number of scale points, where do the cases who are in the center of the distribution mark their scale? If you’re in the planning stages of your thesis or dissertation research, I’d like to work with you to make sure that the simple decisions (like choosing the proper number of rating scale points) are made thoughtfully. It’s vastly easier to work with data from a well-designed study than to cobble together results from a study that rushed forward without making reasoned choices in the design stages.

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