Can you explain how I can be 100% Ireland and Scotland in my Ethnicity estimate on Ancestry?
Why not 99% Ireland and 1% Scotland, how can it be 100% both?
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Ancestry and other companies compare your results with groups of people who are living today in various places, groups called reference populations. They choose people whose families have lived in the same place for a certain amount of time.
Do keep in mind that all these estimates are only that: estimates. I highly recommend the entire series of blog posts that Judy G. Russell has written on the topic, including "Still Not Soup".
For detailed information on how Ancestry arrives at its ethnicity estimates, read their white paper which is available from the Ancestry Support Website.
Other AncestryDNA White Papers are also available. See the support topic AncestryDNA® White Papers.