How many Irish cousins: according to Ancestry – Part 3 in Series

This is Part 3 of a four part series. The full list of posts are:

My previous blog post was on research by scientists at 23andMe on predicting the number of cousins for an Irish person. I applaud 23andMe for the publication of the research in detail.

AncestryDNA have also researched the topic. However it seems that the marketing department release the information with big headline numbers and not a lot of detail.

The Headline Numbers for Ireland

A recent release of information was to mark World DNA day in April 2018. Unfortunately some news outlets reported that the Irish have 14,000 cousins. In contrast, others reported Ancestry as saying that we have 14,000 LIVING cousins up to a distance of 8th cousin. The living distinction is important, but I was more disillusioned on realizing this number is up to 8th degree. I’d like to see the breakdown estimates at each degree, as done by 23andMe, but cannot find the details for Ireland.

British Predictions

AncestryDNA did conduct a detailed study using British birth rates and census data to produce statistics for the average British person. The numbers are shown here. The numbers are lower than in the 23andMe study by Denn et al. The formula used by Denn and Tim X to generate predicted number of cousins is 

If AncestryDNA used the same formula as 23andMe (Denn) and Tim Urban (as discussed in my blog post here), then figuring out the birth rate they utilized is a matter of solving a quadratic equation. I took a shortcut by using symbolab. Plugging in a result of 5 for first cousins and 28 for second cousins, I calculated that if they’d used the formula then their birth rate would have been 2.3. However, solving the same equation of their figures of 3rd and 4th cousins produced different birth rates.

I’m reluctant to repeat numbers for which I can’t explain the provenance. But for the sake of completeness – here are Ancestry’s estimates for British users:AncestryDNA's prediction of the number of Irish cousins

What about Ireland?

Whatever birth rate and formula they used, the Irish birth rate was significantly higher than Britain prior to 1990. So I’m not particularly interested in the British predictions for the purpose of this post. The problem of course is that Ancestry state that they used census data and other statistics going back 200 years for their calculations. Such records are not generally available for Ireland. This is no fault of Ancestry: the sad fact is that Irish records pre-20th century are very patchy).

So it looks like all we have on Ireland from AncestryDNA are their reported calculation of a total of 14,000 1st to 8th living cousins.

How does that compare to the predicted totals from my previous posts – which don’t take into account whether these cousins are living or dead? Hard to say, as I can’t find any details as to how AncestryDNA calculated probability of living. There’s a pattern forming here. Even the ISOGG Wiki which have a page on cousin statistics have to cite the Daily Mirror as their source of AncestryDNA information. With all due respect to the Mirror, it’s a tabloid newspaper as opposed to the peer-reviewed journal that published the 23andMe research.

I do hope that AncestryDNA will produce the level of detail as done by 23andMe. But until then, their estimates are a bit of a bust for me. Knowing the assumed birth rate and other assumptions is important to assess whether the figures realistic for Irish users.

In my next post in this series I’ll discuss some of the assumptions and caveats to be considered. (The list of posts are at the top of this post).



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