What is the appropriate measure of variability for ordinal data?

Answer 1

This is a surprisingly difficult question, partly perhaps because of the ambiguous term 'ordinal'. For instance, horse-race finishes are ordinal--horses usually finish first, second, etc., with no ties; pure order data gives no information about gaps between horses. For such data--the purest form of ordinal data--talk about variability is meaningless. You need data with 'ties' or repeated 'values'--more cases than ordered categories--to talk about variability meaningfully.

If you do have repeated values, one option is to fall back and use nominal variability measures--the Index of Qualitative Variation is one; information statistics also work; and there's always the frequency/percentage table. They don't 'measure' concentration along the categoric order, obviously.

Disappointingly many websites recommend using the range or interquartile range, presumably calculated by assigning numbers to the ordered categories and subtracting. These indices are very dangerous if you assume only qualitative order among categories. This is obviously flawed--if you don't know how far categories are separated, subtracting numbers is flat invalid. For instance, rank states in the US by size--Alaska is 1, RI is 50--and consider the fact that a group from AK, TX, and CA has a range of 2 and a group from NJ and MA has range of 3 [47 - 44]. First, those numbers are really meaningless; second, they sure misrepresent relations among state size differences. Unless you trust that your 'ordinal' categories are pretty close to equal intervals apart--what we call 'quasi-interval'--you simply cannot use range validly to measure ordinal variability. The same reasoning applies to inter-quartile range. You might as well use variance, since describing 'skew' and 'outliers' for ordinal data is very dangerous, itself.

More valid ordinal measures do exist--I cannot recall them. But when you choose an index, take care to examine how it is treating the numbers or other ordering symbols it trades on. Invalidity is rife.