Thurstone scale

In an attempt to approximate an interval level of measurement, psychologist Robert Thurstone developed the method of equal-appearing intervals. This technique for developing an attitude scale compensates for the limitation of the Likert scale in that the strength of the individual items is taken into account in computing the attitude score. It also can accommodate neutral statements.

Constructing the scale

Step 1. Collect statements on the topic from people holding a wide range of attitudes, from extremely favorable to extremely unfavorable. For this example, we will use attitude toward the use of marijuana. Example statements are

It has its place.
Its use by an individual could be the beginning of a sad situation.
It is perfectly healthy; it should be legalized.

Step 2. Duplicates and irrelevant statements are omitted. The rest are typed on 3/5 cards and given to a group of people who will serve as judges.

Step 3. Originally, judges were asked to sort the statements into eleven (11) stacks representing the entire range of attitudes from extremely unfavorable (1) to extremely unfavorable (11). The middle stack is for statements which are neither favorable nor unfavorable (6). Only the end points (extremely favorable and extremely unfavorable) and the midpoint are labeled. The assumption is the intervening stacks will represent equal steps along the underlying attitude dimension. With a large number of judges, for example, using a class or some other group to do the preliminary ratings, it is easier to create a paper-and-pencil version.

Rate each of the following statements indicating the degree to which the statement is unfavorable or favorable to marijuana use. Do not respond in terms of your own agreement or disagreement with the statements; rather, respond in terms of the judged degree of favorableness or unfavorableness. Place an X in the interval that best reflects your judgment. For example: Marijuana is OK for most people, but a few people , may have problems with it.

 1 If marijuana is taken safely, its effect can be quite enjoyable. 2 I think it is horrible and corrupting. 3 It is usually the drug people start on before addiction.

Remind the judges to rate favorability with regard to the target (marijuana), not to give their opinion as whether they agree or disagree with the statement.

Step 4. Each statement will have a numerical rating (1 to 11) from each judge, based on the stack in which it was placed. The number or weight assigned to the statement is the average of the ratings it received from the judges.

 Statement Average rating from 20 judges (11 = extremely favorable) If marijuana is taken safely, its effect can be quite enjoyable. 8.9 I think it is horrible and corrupting. 1.6 It is usually the drug people start on before addiction. 4.9

If the judges cannot rate the item on its favorability or show a high degree of variability in their judgments, the item is eliminated. For example, the statement "Marijuana use should be taxed heavily" was rejected because it was ambiguous. Some judges thought it was pro-marijuana as it implied legalization; others though it was anti-marijuana because it advocated a heavy tax.