NOTE:
OMOP conventions:
Race (this is what we would call ethnicity)
Ethnicity (hispanic vs. not hispanic).

We do not have a concept of hispanic vs. not hispanic in the UK. So all "ethnicity" in CC-HIC OMOP is 0. Race is being used for what we would call ethnicity.

Race is NOT NULL. There is an option in the 2011 census data for "ethnicity not stated". This is an expression of a person not wanting to declare their ethnicity, which is different to us not knowing it. Since this cannot be coded in OMOP, this is all compressed into concept_id 0

Therefore anything missing or declared as us not knowing are now concept_id 0