It's actually from something called the
Likert Scale.
Quote:
A Likert scale (pronounced 'lick-ert') is a type of psychometric response scale often used in questionnaires, and is the most widely used scale in survey research. When responding to a Likert questionnaire item, respondents specify their level of agreement to a statement. The scale is named after Rensis Likert, who published a report describing its use (Likert, 1932).
A typical test item in a Likert scale is a statement. The respondent is asked to indicate his or her degree of agreement with the statement or any kind of subjective or objective evaluation of the statement. Traditionally a five-point scale is used, however many psychometricians advocate using a seven or nine point scale. S. S. Stevens formulated an alternative technology for replacing numeric scales with cross modal matching. such as using the brightness of a light to scale the loudness of a tone. This implicitily uses a continuum or infinite response set for the scale.
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The use of 5-point scales has entered common usage, but if you are designing a survey or questionnaire there are very good reasons for using it.