Supplement missing primary care ethnicity with data from SUS

[chrisjwood16]

Currently we are only using ethnicity recorded in primary care. To ensure we are robustly identifying ethnicity in this project we will supplement missing primary care ethnicity with data from SUS.

This practice is described in the paper Consistency, completeness and external validity of ethnicity recording in NHS primary care records: a cohort study in 25 million patients’ records at source using OpenSAFELY:

It is common practice in OpenSAFELY to supplement primary care ethnicity, where missing, with ethnicity data from SUS [10, 11].

I'm not sure there are ehrQL studies that have done this yet, but a good way to start this issue would be to check https://github.com/opensafely for any usage of the ethnicity_from_sus table.

[milanwiedemann]

Closed in #28