Author Topic: Difference Between Ancestry and MyHeritage cM  (Read 412 times)

Offline 4b2

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Difference Between Ancestry and MyHeritage cM
« on: Monday 03 February 25 16:09 GMT (UK) »
Is anyone able to provide the difference between DNA matches on Ancestry an MyHeritage. i.e. the cM for the same people.

From what I have gathered there is a point around 120cM at which it being to drop from being about equal on the two sites. Then at around 110cM it drops to around an average of about 60% lover on Ancestry.

From my comparisons:

MyHeritageAncestryDifference
24912504101%
283296105%
262275105%
262275105%
213216101%
207210101%
207210101%
176176100%
11811698%
11210594%
1077469%
844756%
725171%
713144%
703753%
604270%
524485%
412971%
371951%
351337%
17953%

Online Spelk

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Re: Difference Between Ancestry and MyHeritage cM
« Reply #1 on: Monday 03 February 25 19:26 GMT (UK) »
I think the cM figures are arrived at by adding together all the blocks of cM. Where the difference comes in is that MyHeritage adds in all the little bits whereas Ancestry does not include the tiny bits.

Offline 4b2

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Re: Difference Between Ancestry and MyHeritage cM
« Reply #2 on: Monday 03 February 25 19:57 GMT (UK) »
I think the cM figures are arrived at by adding together all the blocks of cM. Where the difference comes in is that MyHeritage adds in all the little bits whereas Ancestry does not include the tiny bits.

Ancestry has a system for removing segments that are likely to contribute to false positives. They remove segments of DNA that are commonly found among many individuals, particularly those segments that might not indicate a recent common ancestor but rather shared distant or biogeographical ancestry. This process helps to exclude segments that are overly abundant in certain populations.

Ancestry affects the cM values, particularly for matches below 90 cM. For these lower match levels, Timber can significantly reduce the reported shared DNA if parts of it are deemed not genealogically relevant.

If you put two tests through GedMatch and give is a threshold of 3cM segments, you will get more than what is reported on MyHeritage and Ancestry.

I was looking for for more data to see what the rough point is whereby the cM should be considered differently. As a 40cM match on Ancestry is significant. But a 40cM match could be more like 20cM on Ancestry, and thus could be more in the range of 6C, or a false positive.

Offline Steve3180

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Re: Difference Between Ancestry and MyHeritage cM
« Reply #3 on: Tuesday 04 February 25 12:23 GMT (UK) »
I had a look at this a few years ago and came to the conclusion that there were too many random variables in the calculations to derive anything meaningful. This is what I found -

1. If you use the unweighted shared dna from Ancestry the numbers are much closer.
2. The difference between headline and unweighted figures is random depending on whether it hits an area Ancestry abitrarily deem 'too matchy'.
3. I found references to people who had both tested on MyHeritage and imported their Ancestry test and there were differences. So you would need to know whether the MyHeritage value was tested or imported and they don't tell you.
4. Ancestry and My Heritage use (or used) different chips which test different points in the dna. The unpublished method they use to match the two (imputation) is essentially an educated guess.

You could calculate an average difference given enough data but it wouldn't be of much use in predicting a new value as the underlying process is random.


Offline 4b2

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Re: Difference Between Ancestry and MyHeritage cM
« Reply #4 on: Tuesday 04 February 25 17:43 GMT (UK) »
I had a look at this a few years ago and came to the conclusion that there were too many random variables in the calculations to derive anything meaningful. This is what I found -

Thanks for your input, I was looking at a point at which you can presume there will be a differentiation. From the comparisons I have, it's somewhere after 90-110cM, and although the range can be quite large, multiplying the MyHeritage figure by 0.6 is a decent average. So a seemingly pertinent 60cM, becomes 36cM.

I also feel anything below this 90-110cM range on MyHritage might be a false positive. And I tend to think that about 50% or more of matches in the 20-60cM range are false positives.

Of my Ancestry matches 20cM-self, of the ones that I can parse out the tree for enough generations I'd say the hit rate of finding a relation or what looks to be an NPE is around 80-90%. While the same on MyHeritage is closer to 10-20%. There are useful matches on MyHeritage, but it's a lot more work to flush them out.

Quote
depending on whether it hits an area Ancestry arbitrarily deem 'too matchy'.

On this, I do wonder how often Ancestry might be stripping out chunks of DNA they'd be better off leaving in. On 23AndMe I have one match who is a 4th cousin to my aunt's test and 98cM, yet on Ancestry her mother is just 8cM.

Having gone though over 1,000 matches in multiple tests, I was for quite a while under the impression that either a 2-3X great-grandfather (Breese) might have been born via infidelity. Because there appeared to be zero DNA matches who correlated to any line of his ancestry and overlapped with other relevant matches. It was only after I began going through matches systematically figuring out how all are related that I discovered that my largest cluster of matches, from a 3X great-grandmother (Bennett), had multiple matches who almost all shared two or more lines of ancestry. So they all just got lost in a huge cluster, when in reality there were at least three clusters in there, with 80-90% of them sharing multiple lines. So once I found one connection, they were put aside and I didn't go back and look for more.

Further, on this Breese line I thought may be an NPE, there are some lines that come from it that I'd generally expect to have yielded DNA matches on Ancestry, but as it stands I have zero. Yet I have matches on those lines from MyHeritage, which overlap on segments with closer Breese matches there (i.e. 3rd cousin, 4th cousin).

So in this case I lean to believing that Ancestry has stripped out too much DNA. So, it would be good if they had a feature to also show your DNA matches before Timber has stripped out segments. I imagine the majority of them would be false positives, but it seems likely that some of them would not. You could pull up the results of the ones that have a common ancestor found in the tree, which would probably throw up some more likely connections.

The Timber feature is great, as it does seem strip out all or close to all of the false positives, at least down to 20cM.

I haven't looked into the clusters of matches you can now see with ProTools, below 20cM. So can't say if they are also low/free of false positives. But from what I can see - it seems the smaller the cM of the primary (or largest cM) match in a cluster - the more distant the relationship is likely to be.

I find, for example, you can have a cluster of matches where the largest cM is 70 (a 3rd cousin) and then there are other matches down to 8cM (with relationships as far back as 6th cousin). While if the largest cM match in a cluster is 20-21cM, then the relationships seems to be pushed back a bit further of the conceivable range. i.e. a 21cM match could be a 2C1R, but in these, from what I've seen the relationship is always further back than you'd expect.

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Re: Difference Between Ancestry and MyHeritage cM
« Reply #5 on: Tuesday 04 February 25 17:54 GMT (UK) »
Too many variables between testing platforms, testing procedures and interpretation to draw any conclusions. The only consistent thing about it is the inconsistency.

I only tested with Ancestry but have two matches, 'D' and 'A' who are father and son who have tested with both platforms and I know the origin of the tests on each site. The Timber algorithm doesn't come into the equation as they are all above the 90cM threshold;
 
'D'
Ancestry to Ancestry on Ancestry =319cM
Ancestry to MH on MH =321cM
Ancestry to Ancestry on MH = 371cM

'A'
Ancestry to Ancestry on Ancestry =260cM
Ancestry to MH on MH =317cM*
Ancestry to Ancestry on MH = 326cM*

* are both too high for the h1c2r relationship myself and A actually share based on  the values given on the shared cM project.  Unfortunately for me my two big mystery matches on MH both come out around 300cM and predicted as being a generation above where they likely are in reality. 

My half sibling is a 1759 match on Ancestry but the two Ancestry tests on MH exceed 2000cM.

 

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Re: Difference Between Ancestry and MyHeritage cM
« Reply #6 on: Tuesday 04 February 25 18:38 GMT (UK) »


* are both too high for the h1c2r relationship myself and A actually share based on  the values given on the shared cM project.

The match who is given as 371cM is (seemingly) a 1C2R?

From what I've seen, when you are dealing with removed relationships the shared cM can be a lot larger than one might expect.

One can share up to 2462cM from a grandparent. So there is a very large amount of DNA that can be matched against by people who are 2X removed.

I have a 2C1R with 14cM shared, the smallest suggested by the shared cM project. This further increased my suspicions that there was an NPE on one of our shared lines. But both tests have matches confirming all the lines of ancestry of a 2C1R relationship.

Offline Steve3180

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Re: Difference Between Ancestry and MyHeritage cM
« Reply #7 on: Tuesday 04 February 25 20:57 GMT (UK) »
Quote
I also feel anything below this 90-110cM range on MyHritage might be a false positive. And I tend to think that about 50% or more of matches in the 20-60cM range are false positives.

I do hope not as that would rule out nearly all of my matches on MH. I come from a family with a lot of only children so a tiny number of 2nd, 3rd, and on some lines 4th cousins. Same problem on Ancestry, mostly small matches.

No reason to think the actual direct testing on MH is any more or less accurate than anybody elses, the imputation on imported tests is another matter but as they don't say how they do it it's impossible to judge.

I've just done some calcs for the 31 matches I have on both and the variations are wild -
1. Unweighted to MH - Average 28% more cM on MH, min -30%, max 175%
2. Timber values to MH - Average 56% more on MH, min -2%, max 175%

As to Timber I think the one-size-fits-all approach is a major problem, if they allowed us to choose then fine but making the choice for everyone is very annoying. I have two GG lines that have a high degree of pedigree collapse, and in those Timber strips out a lot of cM. Consequently a lot of small matches will have dropped off the edge.

In general I don't think there are so many false positives, I just think the MRCA is a lot further back than we can look. I saw some work recently that demonstrated that matches under 20cM were more likely to be 10th cousins than 4th and that feels right to me, they're just too far back to find. Even with Timber Ancestry are not saying the stuff they strip out is false just that it matches too many people to be useful.



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Re: Difference Between Ancestry and MyHeritage cM
« Reply #8 on: Wednesday 05 February 25 00:18 GMT (UK) »


* are both too high for the h1c2r relationship myself and A actually share based on  the values given on the shared cM project.

The match who is given as 371cM is (seemingly) a 1C2R?

From what I've seen, when you are dealing with removed relationships the shared cM can be a lot larger than one might expect.

One can share up to 2462cM from a grandparent. So there is a very large amount of DNA that can be matched against by people who are 2X removed.

I have a 2C1R with 14cM shared, the smallest suggested by the shared cM project. This further increased my suspicions that there was an NPE on one of our shared lines. But both tests have matches confirming all the lines of ancestry of a 2C1R relationship.

After a bit of target testing we found my mother is an NPE so that side of my tree is made up exclusively of half relatives and therefore 'A' is a half 1c2r not full. Shared cM gives 269 as the upper limit for that relationship. We can rule out any link through 'A's maternal side bumping the figure up as his mother also tested on both sites but is not a match to me nor are there any mutual shared matches except 'A'. Basically Ancestry tells one story and MH another.