How does Excel calculate its quartiles?
If the data are assumed to be in ascending order, • the lower quartile (Q1) is the ¼(n+3)th observation • the second quartile (median) is the ½(n+1)th observation • the upper quartile (Q3) is the ¼(3n+1)th observation When ¼(n+3), ½(n+1) and ¼(3n+1) are not integers, linear interpolation is used. This is Method 1 of Freund, J and Perles, B (1987) “A New Look at Quartiles of Ungrouped Data”, The American Statistician, 41, 3, 200-203.
