Excel Quartiles - Not so fast...

The formula that I posted for Quartiles in Excel (n+3/4) is not precise. Here's the algorithm for computing quartiles that Excel uses according to the Microsoft knowledge base:

The following is the algorithm used to calculate QUARTILE():

1.	Find the kth smallest member in the array of values, where: k=(quart/4)*(n-1))+1 If k is not an integer, truncate it but store the fractional portion (f) for use in step 3. And where: • • quart = value between 0 and 4 depending on which quartile you want to find • n = number of values in the array
2.	Find the smallest data point in the array of values that is greater than the kth smallest -- the (k+1)th smallest member.
3.	Interpolate between the kth smallest and the (k+1)th smallest values: Output = a[k]+(f*(a[k+1]-a[k])) a[k] = the kth smallest a[k+1] = the k+1th smallest