Assuming:
Index Percentage to use
1 25%
2 50%
3 75%
and number is in column A and index is in column B on same row, like:
A B
104 1
103.65 1
99.25 2
....

In an empty column, in same row (we'll use row 1 here for example) enter
=IF(B1=1,A1*.25,IF(B1=2,A1*.50,IF(B1=3,A1*.75,"")) )
extend that formula down the column to match entries in columns A/B.
Substitute actual colum letters for A and B in the formula.


"Dan" wrote:

Hi, I have a large series of numbers saved in one column
[34,5,21,98,34,3,432,55,27,90,1125,......]. These data are indexed, e.g. by
[1,1,1,1,1,1,2,2,2,2,2,2,2,2,2,......] which are in another column.

I need to calculate percentiles (25%,50% and 75%) of these numbers for
different indexes. In another word, I need to calculate percentiles for those
numbers with index of 1, 2, ... respectively, not the percentiles of all the
numbers.

Thanks a lot!