If I've misinterpreted your problem, let me know. I'm thinking that perhaps
you want to calculate what percentage of the numbers with those indexes are
in comparison to the total number of all that are indexed?

"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!