When interviewer-based surveys try to gather data on ethnicity their questions may not always capture what they are aiming for; rather, it is the interviewees’ opinion on what ethnic group (or groups) they think they belong to.
Partly in response to this sort of experience, the idea of ethnicity has now been replaced by today’s ‘gold standard’ democratic definition – self-declared ethnic affiliation. In short, you are who you say you are. This may or may not allow people to nominate a mixed or multiple group membership depending on which form you are filling in. Also, your declaration is not subject to approval from the group(s) you claim to belong to. This is the current New Zealand Standard Ethnicity definition.
Geoff Chambers and Paul Callister, “DNA tests are all fine and dandy, but they can never tell us who we really are,” The Dominion Post, April 3, 2018. https://www.stuff.co.nz/dominion-post/comment/102707496/DNA-tests-are-all-fine-and-dandy-but-they-can-never-tell-us-who-we-really-are.