The census contains a question "What is your ethnic group?"
Document
Scotland Census Specimen (544.28 KB / PDF)
Because the question in the census asked people to classify their ethnicity themselves we believe it to be the best information available.
SHELS linked census records to health information eg death records; the cancer register and others and data that is collected about our admissions to hospitals . This allowed the relationship between ethnicity and health issues affecting the population of Scotland to be examined.