Central Tendency
The median, i.e. middle-ranked, item is allowed as the measure of central tendency; however, the mean (or average) as the measure of central tendency is not allowed. The mode is allowed.
In 1946, Stevens observed that psychological measurement, such as measurement of opinions, usually operates on ordinal scales; thus means and standard deviations have no validity, but they can be used to get ideas for how to improve operationalization of variables used in questionnaires.
Most psychological data collected by psychometric instruments and tests, measuring cognitive and other abilities, are of the interval type, although some theoreticians have argued they can be treated as being of the ratio type (e.g. Lord & Novick, 1968; von Eye, 2005). However, there is little prima facie evidence to suggest that such attributes are anything more than ordinal (Cliff, 1996; Cliff & Keats, 2003; Michell, 2008). In particular, IQ scores reflect an ordinal scale, in which all scores are meaningful for comparison only. There is no absolute zero, and a 10-point difference may carry different meanings at different points of the scale.
Read more about this topic: Numerical Data, Ordinal Scale
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