Take up IPA proposal

[AramZS]

The Proposal repo holds issue 2 which intends to cover private attribution processes to "addresses the aggregate attribution measurement use case". We should take up this proposal, decide next steps and move forward with it as determined by the group.

[martinthomson]

Hi @AramZS, perhaps the plan I suggested in #9 (comment) is a neater approach than considering just one of the many proposals - at this time. I don't think we need to do anything more at this stage than ask what functionality is being considered (perhaps attribution, maybe reach and frequency) and the way in which we might approach providing that functionality.

[AramZS]

Merging this with patcg/patcg.github.io#5 to be clearer that the intent here is to work towards a privacy-respecting measurement proposal that takes into account all of the use cases with a mind towards understanding each proposal's functionality and how that functionality might be approached in a more general sense with an eye towards producing one or more unified outputs.