Likert scaling

A Likert scale (/ˈlɪk.ərt/ LIK-ərt but more commonly pronounced /ˈlaɪ.kərt/ LY-kərt) is a psychometric scale commonly involved in research that employs questionnaires. It is the most widely used approach to scaling responses in survey research, such that the term (or more accurately the Likert-type scale) is often used interchangeably with rating scale, although there are other types of rating scales.

The scale is named after its inventor, psychologist Rensis Likert. Likert distinguished between a scale proper, which emerges from collective responses to a set of items (usually eight or more), and the format in which responses are scored along a range. Technically speaking, a Likert scale refers only to the latter. The difference between these two concepts has to do with the distinction Likert made between the underlying phenomenon being investigated and the means of capturing variation that points to the underlying phenomenon.

When responding to a Likert item, respondents specify their level of agreement or disagreement on a symmetric agree-disagree scale for a series of statements. Thus, the range captures the intensity of their feelings for a given item.

A scale can be created as the simple sum of questionnaire responses over the full range of the scale. In so doing, Likert scaling assumes distances between each item are equal. Importantly, "All items are assumed to be replications of each other or in other words items are considered to be parallel instruments" (p. 197). By contrast, modern test theory treats the difficulty of each item (the ICCs) as information to be incorporated in scaling items.

...
Wikipedia