My last reply does not appear to have communicated with you, sorry.

The interquartile range is calculated using the 1st & 3rd sample quartiles,
but there are various ways to calculate those quartiles.

Five of Hyndman and Fan's sample quantile definitions have a particularly
simple common form given by the following VBA code.  You select among the
methods according to which definition of m you uncomment.

Function quantile(data, p)
' p=fraction of population, e.g. p=0.25 for 1st quartile
   n = WorksheetFunction.Count(data)
'   m = 0       ' H&F 4: SAS (PCTLDEF=1), R (type=4), Maple (method=3)
'   m = 0.5     ' H&F 5: R (type=5), Maple (method=4)
'   m = p       ' H&F 6: Minitab, SPSS, BMDP, JMP, SAS (PCTLDEF=4), R
(type=6), Maple (method=5)
'   m = 1 - p   ' H&F 7: Excel, S-Plus, R (type=7[default]), Maxima, Maple
(method=6)
'   m = (p+1)/3 ' H&F 8: R (type=8), Maple (method=7[default])
'   m=(p+1.5)/4 ' H&F 9: R (type=9), Maple (method=8)
   npm = n * p + m
   j = Fix(npm):   If j = 0 Then j = 1
                   If j > n Then j = n
   g = npm - j
   quantile = WorksheetFunction.Small(data, j)
   If g >= 0 And j < n Then
       quantile = (1 - g) * quantile + g * WorksheetFunction.Small(data, j
+ 1)
   End If
End Function

Excel, S-Plus, etc use H&F definition 7, which returns SMALL(data,i) as
quantile(data,(i-1)/(n-1)) and interpolates in between.  For a continuous
distribution, this will tend to give too narrow an interquartile range, since
there will tend to be a small fraction of the population beyond the extreme
sample observations.  In particular, for odd n (=2*k+1), Excel calculates the
1st (3rd) quartile as the median of the lower (upper) "half" of the sample
including the sample median (k+1 observations).

Minitab, etc use H&F definition 6, which calculates the 1st (3rd) quartile
as the median of the lower (upper) "half" of the sample.  This "half" sample
excludes the sample median (k observations) for odd n (=2*k+1).  This will
tend to be a better estimate for the population quartiles, but will tend to
give quartile estimates that are a bit too far from the center of the whole
sample (too wide an interquartile range).

Hyndman and Fan recommend their definition 8 (Maple's default definition),
which gives quartiles between those reported by Minitab and Excel.  This
approach is approximately median unbiased for continuous distributions.

Jerry