Spurious Accuracy in DNA Comparison

by Paddy Waldron

Last updated: 20 July 2022

URL: http://pwaldron.info/DNA/SpuriousAccuracy.html

The various DNA companies peddling estimated ethnicity percentages continue to damage their reputations in the eyes of anyone who has a basic understanding of mathematics or statistics by indulging in spurious accuracy. I found the following nice definition of spurious accuracy in an accounting textbook:
a pretence to precision that is either unattainable or useless (or both).
As of 20 July 2022, the 5th longest half-identical region which I share with a MyHeritage match to whom I have not established my precise genealogical relationship is with Robert, who now has two different kits at MyHeritage.

MyHeritage recognises that these two kits represent the same person:

50cM match with
43cM match with
However, MyHeritage makes no attempt to aggregate the information in the two kits, instead presenting separate match information, with differences that some might find surprising.
The first obvious difference is in the estimated amount of DNA shared with me: 43.3 centiMorgans v. 50.3 centiMorgans. The large half-identical region of 43.3cM is found in both kits, but a second half-identical region of 7.0cM is found in only one of the kits.

More striking are the differences in the estimated ethnicity percentages:

There are several possible explanations for the differences in the data extracted from the same person's DNA:

The differences between the estimated ethnicity percentages can be as high as 22%, here in the case of Iberian, which has the third-highest estimated ethnicity percentage for one kit but only the fifth-highest percentage for the other:

Ethnicity 43.3cM kit 50.3cM kit Ratio
Irish, Scottish, and Welsh 54.3 49.5 0.91
English 23.2 27.5 1.19
Italian 8.5 8.1 0.95
Baltic 7.3 6.7 0.92
Iberian 6.7 8.2 1.22

Presenting these estimated ethnicity percentages to three significant digits (or one tenth of a percentage point), with no reference to the associated margins of error, will undoubtedly cause many customers to treat them as far more accurate than they actually are.  In the words of the accountant's definition, the DNA companies are undoubtedly guilty of a pretence to precision that is both unattainable and useless.

The DNA companies displaying smaller geographical areas under titles such as "Genetic Groups" do appear to realise that it is impossible to quantify precisely the proportion of a customer's ancestry attributable to these smaller areas.  It is still a surprise to see that the presence or absence of such smaller areas is so sensitive to what should be just measurement error.  In these case, two different Genetic Groups appear or disappear completely for no apparent reason.