When I read there was a proposed standard for statistical metadata, the first questions that came to mind were what is the relationship between StatDCAT-AP and:
The first two are mentioned in Draft 4, but not the latter. In fact, the official statistics institute of Brazil (IBGE) has adopted the DDI standard for statistical datasets. It would be good to harmonize these standards into StatDCAT-AP, since we are considering adpoting DCAT-AP for the national open data portal.
I'd like to mention, also, that there is a CKAN extension named DDI Importer/Harvester, made by Liip/Switerland and The World Bank. Given that statistical data discoverability is stated as an objective of StatDCAT-AP, and that there might be already an overlap between users of DCAT-AP and DDI, I think it is important to consider DDI in the context of the StatDCAT-AP application profile.