An INTERVAL scale is one in which intervals at different points on the scale are equal. Examples are the Celsius and Fahrenheit temperature scales. The difference between 20 and 22 degrees is the same as the difference between 15 and 17 degrees. Many psychologists would also treat STANDARDISED psychological measures, for example of 'neuroticism and IQ' Everitt & Wykes (1999, p. 42) as interval scales, although the legitimacy of doing so has been debated. For more on this, see ORDINAL data for a discussion of important issues concerning psychological measures.
A RATIO scale is similar to an interval scale except that whereas the zero point in an interval scale is arbitrary, the ratio scale has a true zero point. Temperature measured in degrees Kelvin is a ratio scale. It has a true zero point, whereas the zero point on the Celsius scale is arbitrarily placed as the freezing point of water. Other substances have different freezing points, and any of them or none could have been chosen. Quantities such as milligrams of alcohol consumed in a day, or hours of work done on a task, height and weight are also ratio scales.
Everitt & Wykes (1999). A Dictionary of Statistics for Psychologists. London, Arnold.
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