Different results with different companies

[LizzieL]

I tested with Ancestry and uploaded my DNA recults to My Heritage, FT-DNA and Gedmatch.
Several of my matches have done the same. Consistently the number of cM match is slightly greater on other platforms than Ancestry due to the fact that Ancestry will truncate the results.

However I recently noticed another anomaly.
A second cousin once removed tested with MH a few years ago. Our match is 152.9 over 9 segments. Longest 22.6cM

A few weeks ago he tested with Ancestry (has to be a new teat, because you can't upload data to Ancestry). With Ancestry our match was 158cM over 10 segments. Longest 29cM.
As Ancestry has no chromosome browser, I can't see where the extra segment comes from.

Has anyone else tested with two companies and got  different results?

[Gadget]

The segments anomaly is because Ancestry discount some segments to allow for matches by locality.  They've done this  for as long as I can remember.  This is also the reason for the difference in the  shared cMs.

I recall that there are a number of threads on these anomalies.


Gadget

[LizzieL]

But if Ancestry discount segments it should be lower than MH, but the Ancestry test has 10 segments and MH 9 segments. Everything in Ancestry is higher:  total cM, longest segment.

[Gadget]

It could be changes in the method they use over time. Could you ask him to upload his lates test to My Heritage  and compare again.

[phil57]

You will also find anomalies because, as far as I'm aware, no single company samples 100% of your DNA for consumer market autosomal testing. One test might include lengths that another may miss, and vice versa. GEDmatch allows you to create a superkit, by merging tests from different companies. Interestingly, this can often lower the length of your matches because it allows some string lengths to be either matched or ruled out, where they may instead have been inferred as false matches in the results of a single test.

[melba_schmelba]

You will also find anomalies because, as far as I'm aware, no single company samples 100% of your DNA for consumer market autosomal testing. One test might include lengths that another may miss, and vice versa. GEDmatch allows you to create a superkit, by merging tests from different companies. Interestingly, this can often lower the length of your matches because it allows some string lengths to be either matched or ruled out, where they may instead have been inferred as false matches in the results of a single test.

This is exactly correct and not something that many people understand when buying the tests. These consumer tests are not Whole Genome Sequencing which although getting cheaper will set you back at least £400 - I made a thread on this here
https://www.rootschat.com/forum/index.php?topic=861091.0

two videos about Superkits here, which allow you to minimize the gaps on GEDMATCH

https://www.youtube.com/watch?v=2DO46bW5YAc
https://www.youtube.com/watch?v=IJmAHNSODuw

23andme kits since 2017, and MyHeritage and FTDNA kits since 2019, and LivingDNA pre 2019, all based on the GSA chip will all provide very inaccurate matching on GEDMATCH unless you combine with an Ancestry DNA kit, or an older kit from the first three companies due to very low overlap with the GEDMATCH database SNPs.