A Likert scale ( LIK-ərt,) is a psychometric scale named after its inventor, American social psychologist Rensis Likert, which is commonly used in research questionnaires. It is the most widely used approach to scaling responses in survey research, such that the term (or more fully the Likert-type scale) is often used interchangeably with rating scale, although there are other types of rating scales. 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 former. 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 Likert scale can be scored as the simple sum or average of its constituent Likert item scores. In so doing, Likert scaling assumes distances between each choice (answer option) are equal. Many researchers employ a set of such items that are highly correlated (that show high internal consistency) but also that together will capture the full domain under study (which requires less-than perfect correlations). Others hold to a standard by which "All items are assumed to be replications of each other or in other words items are considered to be parallel instruments". By contrast, modern test theory treats the difficulty of each item (the ICCs) as information to be incorporated in scaling items.
Composition
A Likert scale consists of several Likert items, where a composite (total) score typically is computed as the sum (or average) of the item scores. Because many Likert scales pair each constituent Likert item with its own instance of a visual analogue scale (e.g., a horizontal line, on which the subject indicates a response by circling or checking tick-marks), an individual item is itself sometimes erroneously referred to as being or having a scale, with this error creating pervasive confusion in the literature and parlance of the field. A Likert item is simply a statement that the respondent is asked to evaluate by giving it a quantitative value on any kind of subjective or objective dimension, with level of agreement/disagreement being the dimension most commonly used. Well-designed Likert items exhibit both "symmetry" and "balance". Symmetry means that they contain equal numbers of positive and negative positions whose respective distances apart are bilaterally symmetric about the "neutral"/zero value (whether or not that value is presented as a candidate). Balance means that the distance between each candidate value is the same, allowing for quantitative comparisons such as averaging to be valid across items containing more than two candidate values. The format of a Likert item typically is composed of five or seven response options. The five-response-option item originally proposed by Rensis Likert was:
Strongly disapprove Disapprove Undecided Approve Strongly approve However, this eventually was replaced by "degree of agreement" response options; e.g.,
Strongly disagree Disagree Neither agree nor disagree Agree Strongly agree A Likert item with seven response options adds an additional pair of response options (e.g., "slightly agree" and "slightly disagree," "somewhat agree" and "somewhat disagree," or "moderately agree" and "moderately disagree"). Likert scaling is a bipolar scaling method, measuring either positive or negative response to a statement. Sometimes even-point response options are used, where the middle option of "neither agree nor disagree" is not available. This is sometimes called a "forced choice" method, since the neutral option is removed. The neutral option can be seen as an easy option to take when a respondent is unsure, and so whether it is a true neutral option is questionable. A 1987 study found negligible differences between the use of "undecided" and "neutral" as the middle option in a five-point Likert item. Likert scales may be subject to distortion from several causes. Respondents may:
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