| Abstract: | Two methods of constructing equal-interval scales for educational achievement are discussed: Thurstone's absolute scaling method and Item Response Theory (IRT). Alternative criteria for choosing a scale are contrasted. It is argued that clearer criteria are needed for judging the appropriateness and usefulness of alternative scaling procedures, and more information is needed about the qualities of the different scales that are available. In answer to this second need, some examples are presented of how IRT can be used to examine the properties of scales: It is demonstrated that for observed score scales in common use (i.e., any scores that are influenced by measurement error), (a) systematic errors can be introduced when comparing growth at selected percentiles, and (b) normalizing observed scores will not necessarily produce a scale that is linearly related to an underlying normally distributed true trait. |
