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!