The following is extracted from one of my webpage. Hope it can help:
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The issue regarding the appropriateness of ordinal-scaled data in
parametric tests was unsettled even in the eyes of Stevens (1951), the
inventor of the four levels of measurement: "As a matter of fact, most of
the scales used widely and effectively by psychologists are ordinal
scales ¡K there can be involved a kind of pragmatic sanction: in numerous
instances it leads to fruitful results." (p.26) Based on the central
limit theorem and Monte Carlo simulations, Baker, Hardyck, and
Petrinovich (1966) and Borgatta and Bohrnstedt (1980) argued that for
typical data, worrying about whether scales are ordinal or interval
doesn't matter.
Another argument against not using interval-based statistical techniques
for ordinal data was suggested by Tukey (1986). In Tukey's view, this was
a historically unfounded overreaction. In physics before precise
measurements were introduced, many physical measurements were only
approximately interval scales. For example, temperature measurement was
based on liquid-in-glass thermometers. But it is unreasonable not to use
a t-test to compare two groups of such temperatures. Tukey argued that
researchers painted themselves into a corner on such matters because we
were too obsessed with "sanctification" by precision and certainty. If
our p-values or confidence intervals are to be sacred, they must be
exact. In the practical world, when data values are transformed (e.g.
transforming y to sqrt(y), or logy), the p values resulted from different
expressions of data would change. Thus, ordinal-scaled data should not be
banned from entering the realm of parametric tests. For a review of the
debate concerning ordinal- and interval- scaled data, please consult
Velleman and Wilkinson (1993).
from:
http://seamonkey.ed.asu.edu/~alex/teaching/WBI/parametric_test.html
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Chong-ho (Alex) Yu, Ph.D., MCSE, CNE
Academic Research Professional/Manager
Educational Data Communication, Assessment, Research and Evaluation
Farmer 418
Arizona State University
Tempe AZ 85287-0611
Email: [EMAIL PROTECTED]
URL:http://seamonkey.ed.asu.edu/~alex/
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