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A rating scale is a set of categories designed to elicit information about a quantitative attribute in social science. Common examples are the Likert scale and 1-10 rating scales for which a person selects the number which is considered to reflect the perceived quality of a product.
In Psychometrics, rating scales are often referenced to a statement which expresses an attitude or perception toward something. The most common example of such a rating scale is the Likert scale, in which a person is asked to select a category label from a list indicating the extent of disagreement or agreement with a statement.
The basic feature of any rating scale is that it consists of a number of categories. These are usually assigned integers. For example, an example of the use of a Likert scale is as follows.
- Statement: I could not live without my iPod.
- Response options:
- 1. Strongly Disagree
- 2. Disagree
- 3. Agree
- 4. Strongly Agree
It is common to treat the numbers obtained from a rating scale directly as measurements by calculating averages, or more generally any arithmetic operations. Doing so is not however justified. In terms of the levels of measurement proposed by S.S. Stevens, the data are ordinal categorisations. This means, for example, that to agree strongly with the above statement implies a more favourable perception of iPods than does to agree with the statement. However, the numbers are not interval-level measurements in Stevens' schema, which means that equal differences do not represent equal intervals between the degree to which one values iPods. For example, the difference between strong agreement and agreement is not necessarily the same as the difference between disagreement and agreement. Strictly, even demonstrating that categories are ordinal requires empirical evidence based on patterns of responses (Andrich, 1978).
More than one rating scale is required to measure an attitude or perception due to the requirement for statistical comparisons between the categories in the polytomous Rasch model for ordered categories (Andrich, 1978). In terms of Classical test theory, more than one question is required to obtain an index of internal reliability such as Cronbach's alpha (Cronbach, 1951) which is a basic criterion for assessing the effectiveness of a rating scale and, more generally, a psychometric instrument.
- Cronbach, L. J. (1951). Coefficient alpha and the internal structure of tests. Psychometrika, 16, 297-333.
- Andrich, D. (1978). A rating formulation for ordered response categories. Psychometrika, 43, 357-74.
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