Statistical Question

[R.C. Intrieri]

Dear Tipsters,

I am attempting to explain the formula to calculate the Median from Grouped data.
The formula for this is the following

Md = Lm + w [(n/2 � fcum)/fm]
Lm = lower limit of the interval that contains the median.
W = width of the interval
fm = is the frequency
fcum = the number of observations falling below this interval.

The formula comes from Glass & Hopkins (3rd edition) Statistical methods in education and psychology
EQ 4.1 P.52

The question that I have (and I need a reference if possible) is what is the justification from a theoretical point of view for using "the number of observations falling below the interval" as opposed to using the number of observations falling above the interval?  Any help would be greatly appreciated.  Thanks.

Bob Intrieri
Robert C. Intrieri, PhD         Office (309) 298-1336   
Department of Psychology        Fax    (309) 298-2179
Western Illinois University     E-mail [EMAIL PROTECTED]
1 University Circle
Macomb IL 61455-1390

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