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Surveys are consistently used to measure quality. For example, surveys might be used to gauge customer perception of product quality or quality performance in service delivery.
Likert scales are a common ratings format for surveys. Respondents rank quality from high to low or best to worst using five or seven levels.
Statisticians have generally grouped data collected from these surveys into a hierarchy of four levels of measurement:
1. Nominal data: The weakest level of measurement representing categories without numerical representation.
2. Ordinal data: Data in which an ordering or ranking of responses is possible but no measure of distance is possible.
3. Interval data: Generally integer data in which ordering and distance measurement are possible.
4. Ratio data: Data in which meaningful ordering, distance, decimals and fractions between variables are possible.
Data analyses using nominal, interval and ratio data are generally straightforward and transparent. Analyses of ordinal data, particularly as it relates to Likert or other scales in surveys, are not. This is not a new issue. The adequacy of treating ordinal data as interval data continues to be controversial in survey analyses in a variety of applied fields.1,2
An underlying reason for analyzing ordinal data as interval data might be the contention that parametric statistical tests (based on the central limit theorem) are more powerful than nonparametric alternatives. Also, conclusions and interpretations of parametric tests might be considered easier to interpret and provide more information than nonparametric alternatives.
However, treating ordinal data as interval (or even ratio) data without examining the values of the dataset and the objectives of the analysis can both mislead and misrepresent the findings of a survey. To examine the appropriate analyses of scalar data and when its preferable to treat ordinal data as interval data, we will concentrate on Likert scales.
Basics of Likert Scales
Likert scales were developed in 1932 as the familiar five-point bipolar response that most people are familiar with today.1 These scales range from a group of categories-least to most-asking people to indicate how...