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:
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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 |

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