When using the Thurstone differential method, interviewees are asked to select those statements with which they agree from a list of between 20 and 25 statements. The statements are derived from an original list of 100 to 200 statements that are evaluated by a panel of 15 to 20 judges. Each judge is asked to place those statements in 11 nearly equal piles. These piles represent the judgements of the panel members as to which statements are more favourable and which are least favourable about the subject. The sixth or middle pile is the neutral position.
Mean scores for each of the original 100 to 200 statements are determined on the basis of the piles in which they were placed by the judges. The 20 to 25 statements with the smallest dispersion are chosen to be included in the survey. From this latter list the interviewee is asked to check only those with which they agree, enabling a mean score for each interviewee to be obtained. This score quantifies the attitude of each interviewee towards the subject under study. Many people argue that the scale is an ordinal rather than interval scale.
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