iif(grp_change<1,1,0) seems to be correct in the sense that in starting it counts wrong for few pages but gradually counts ok. i.e. the data of very first customer contains 3 pages, what it does start with 1 and in 2nd page it gives 2 and in 3rd, 4th gives same 2. And for the next customer it starts again with No.2 and so on.

Please suggest.