So how accurate are these DNA ethnicity tests?
Here is a rough estimate of my father's actual ancestry, based on the paper trail:
* The Colonial American ancestry is too difficult to break down further but it consists of Scottish, English, Irish, German, African, and Native American
** The Québécois ancestry has a trace of Italian and Portuguese but mostly traces back to various regions in France
If you are reading the above colour-coded chart correctly, you'll see that my grandmother, Grace, is 50% Scottish, 25% Irish, 12.5% Québécois , 12.5% Cornish. She's clearly very Celtic, which is probably why Ancestry.com DNA gave my father a 43% Irish (Native Briton) percentage.
Here is the 23andme speculative breakdown for my father's ethnicity. I've moved the paper trail percentages into the 23andme category where I feel it fits best.
So 23andme is far more precise than I could be, although if I were to guess at the actual percentages in Colonial American, it would be very close to what 23andme suggested.
The French and German category is obviously too low but German is notoriously hard to categorize. For example, English people have significant amounts of German DNA from the Angles and Saxons. A lot of this German DNA probably ends up in the Broadly Northwest European for that reason.
All-in-all, it looks like 23andme does a pretty good job with ethnicity testing. I'm actually surprised at the precision and accuracy. Of course, 23andme has a huge database of DNA from western Europe so perhaps I shouldn't have been. People with ancestries from other parts of the world are probably less impressed.