Previous week in a pivot having week as a dimension

[googel84]

Good evening Community, I built a pivot table 8 dimensions and a series of indicators in order to start analyzing merchandising data coming from agents' inspections. Among these dimensions there is also annee_semaine_ref, that represents the reference year/week. I put it at the end of my pivot and on top of the others so that values are spread horizontally.

Among indicators I already have one (named Nb ref Obl) that counts the number of mandatory products (given an assortiment) that the agents should have detected and I can see this number for each of annee_semain_ref value. Now, my goal is to create a new indicator (named Nb ref Obl semaine prec.) that represents the same things but related to the previous week. I tried a couple of set-analysis and I also looked at quite a lot of posts but without finding the answer.

The problem is that the selection made by a pivot is stronger then my set-analysis and I can't find the syntax to bypass it, i.e. to make the indicator evaluate to something different from zero.

My expression, at the end of all my tries, is the following simple one:

=count({1<ass_obl_fac={'O'},annee_semaine_ref={'2011/44'}>} produit_cod)

At this point I cabled the week value but I know I can calculate it.

Which is the set-analysis syntax to use?

Thanks in advance.