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:<math>\frac{\frac{{{Q}(3/4)}+{{Q}(1/4)}}{2}-{{Q}(1/2)}}{\frac{{{Q}(3/4)}-{{Q}(1/4)}}{2}}
=\frac{{{Q}(3/4)}+{{Q}(1/4)}-2{{Q}(1/2)}}{{{Q}(3/4)}-{{Q}(1/4)}},</math>
where ''Q'' is the [[quantile function]] (i.e., the inverse of the [[cumulative distribution function]]). The numerator is difference between the average of the upper and lower quartiles (a measure of location) and the median (another measure of location), while the denominator is the [[semi-interquartile range]] <math>({Q}(3/4)}-{{Q}(1/4))/2</math>, which for symmetric distributions is equal to the [[Average absolute deviation|MAD]] measure of [[statistical dispersion|dispersion]].{{citation needed}}
Other names for this measure are Galton's measure of skewness,<ref name=Johnson1994>{{harvp|Johnson, NL|Kotz, S|Balakrishnan, N|1994}} p. 3 and p. 40</ref> the Yule–Kendall index<ref name=Wilks1995>Wilks DS (1995) ''Statistical Methods in the Atmospheric Sciences'', p 27. Academic Press. {{isbn|0-12-751965-3}}</ref> and the quartile skewness,<ref>{{Cite web|url=http://mathworld.wolfram.com/Skewness.html|title=Skewness|last=Weisstein|first=Eric W.|website=mathworld.wolfram.com|language=en|access-date=2019-11-21}}</ref>
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