I did my own test on the validity of large segments of DNA matches on MyHeritage. I had a personal pile-up on chromosome 16. There were 30 matches which were in the range of 25 to 30 cM in length which overlapped each other. I tested each set to determine which ones were from my Paternal chromosome, and which were from my Maternal chromosome.

I had two known paternal cousins which had matches in that region, and non of the other 28 triangulated with both of them. So I only had two valid Paternal matches.
There were five who matched each other (triangulated) and they were presumed Maternal matches. None of them were known relatives and the total match was too small to expect to find the relationship through genealogical trees.

The other 23 large matches did not triangulate with anyone. These were all false matches, or what I prefer calling, pseudomatches.

The final determination is that in MyHeritage, over 76% of the large (bigger than 25 cM) matches are false matches. They are not real. This is undeniable data.

--
Bob Smiley
Kirkland, Washington USA

Endogamy is a challenge but for those of us who have only one Ashkenazi relationship, in my case my father’s mother, I find my small Ashkenazi DNA (27%) is more accurate and much easier to connect. I am connected to almost everyone from Raudnitz an der Elbe, N Bohemia. So do not overlook these small matches for those who are not 100% Jewish DNA.

Dorann Jacobs Cafaro
Charleston, SC
732 687-5318

Researching Grauer, Dray, Marcus, May, Weisbach, Hecht, Cohen, Kahn
(Dray being a brick wall)

My matches with HM on MyHeritage, FTDNA, and GEDmatch. All I know for sure is that 60+ cM segment didn't come from my mother, who doesn't match HM at all on chromosome 7 (with a 3 cM minimum)
Yellow - Largest estimate for a given segment
Blue - Segment only reported by one site

 

MyHeritage
Chromosome	Start Location	End Location	Start RSID	End RSID	Centimorgans	SNPs
6	1.056E+08	112518090	rs1417736	rs7738951	8.7	3584
6	1.516E+08	1.603E+08	rs4489165	rs220729	13.5	5632
7	4776780	46390788	rs75662232	rs898930	62.2	26368
7	67594586	82373405	rs7777871	rs35847872	17.3	6656
16	88165	2908703	rs2541696	rs8046218	6.1	1920
22	36604605	41736267	rs132744	rs5751080	7.8	2816
FTDNA
Chromosome	Start Location	End Location	Centimorgans	Matching SNPs
6	1.063E+08	112621250	7.94	2370
6	1.51E+08	1.601E+08	14.86	3984
7	4940038	47102645	64.32	18302
7	68622989	83429355	16.03	4575
13	81485364	91104009	6.02	2546
14	35853388	44331104	6.50	2412
22	36639742	44744200	11.96	3146
GEDmatch
Chr	B37 Start Pos'n	B37 End Pos'n	Centimorgans (cM)	SNPs	Segment threshold	Bunch limit	SNP Density Ratio
2	2.274E+08	2.319E+08	7.2	722	201	120	0.28
6	1.056E+08	112345811	8.3	1177	222	133	0.36
6	1.515E+08	1.599E+08	12.7	1704	190	114	0.33
7	4646190	46618644	61.2	8710	181	108	0.34
7	75059630	83311410	10.5	1470	191	114	0.35
14	30714764	33709003	8.1	418	199	119	0.25
22	36570688	45118533	14.7	1174	192	115	0.25

--
Steven Usdansky
usdanskys@...
USDANSKY (Узданский): Turec, Kapyl, Klyetsk, Nyasvizh, Slutsk, Grosovo
SINIENSKI: Karelichy, Lyubcha, Navahrudak
NAMENWIRTH: Bobowa, Rzepiennik
SIGLER: "Minsk"

As a follow-up to my post and the suggestion of several posts, I was able to compare the results of match data for my mother and another person (Mary) at MyHeritage, FTDNA and GEDMatch.  

MyHeritage

Chromosome	Start Location	End Location	Start RSID	End RSID	Centimorgans	SNPs
1	109759998	151341570	rs2924	rs41284998	25.2	7552
2	79935344	89125131	rs76042919	rs2012201	8.3	3712
4	37349537	55823638	rs13120150	rs7684211	14.1	7168
5	149420698	167290121	rs2304070	rs116611957	20.2	9600
10	72680375	79844239	rs7079039	rs7100515	6.9	3968
10	108448200	115522272	rs1252035	rs2419878	7.6	3968
11	36423369	63143137	rs12290256	rs201479912	15.2	10368
12	5906873	12094208	rs6489659	rs78137435	11.7	2688
18	11225618	19261413	rs12457940	rs11662721	6.3	2048
20	53357605	56460394	rs74422090	rs6099816	8.3	2304

FamilyTree DNA

Chromosome	Start Location	End Location	Centimorgans	Matching SNPs
1	109727832	113810931	6.38	1123
2	79803287	88999248	8.68	1436
4	37250289	55984959	15.47	2816
5	149294495	159073110	9.55	2197
10	108316278	115620108	8.01	1649
11	36376603	44312566	6.71	1419
12	5775875	12208613	11.94	1718
18	11229655	19485946	6.91	847
20	54409617	56478627	6.17	831

GEDMatch

Chr	B37 Start Pos'n	B37 End Pos'n	Centimorgans (cM)	SNPs	Segment threshold	Bunch limit	SNP Density Ratio
4	37,250,289	49,061,848	12.4	1,721	202	121	0.32
5	149,301,335	158,958,254	10.2	1,630	195	117	0.32
10	108,859,104	115,614,740	8.1	1,164	198	118	0.33
11	36,412,655	44,311,968	7	1,095	196	117	0.3
12	6,009,595	12,208,578	12.1	1,226	184	110	0.37
20	53,438,513	56,478,015	7.8	884	195	117	0.4

Some idea about where the discrepancy occurs came from a note in the FTDNA report:  the section on Chr 1 where MyHeritage reported a 25.2cM segment match was marked by FTDNA as a "SNP poor region: not tested for Family Finder."  

The bottom line for my purposes is that the MH report in this case was misleading and a more realistic reading  (3 out of 4 test sites) suggests this match falls below the threshold where I could reasonably expect to find a family tree connection.  I do want to point out that in the other matches I reviewed, MyHeritage was often within range of the other services' results and my experience in this case does not mean that MH testing should be deprecated generally.  I also want to note that I was fortunate to have the cooperation of the other test taker, who engaged in an exchange to explore a possible connection, shared pointers to other sites where she was tested and provided access to her family tree.  Thanks also to those who responded to my post.

Lee David Jaffe
===============
Surnames / Towns:  Jaffe / Suchowola and Bialystok, Poland ; Stein (Sztejnsapir) / Bialystok and Rajgrod ; Roterozen / Rajgrod ; Joroff (Jaroff, Zarov) / Chernigov, Ukraine ; Schwartz (Schwarzstein) / Ternivka, Ukraine ;  Weinblatt / Brooklyn, Perth Amboy, NJ ; Koshkin / Snovsk, Ukraine ; Rappoport / ? ; Braun / Wizajny, Suwalki,  Ludwinowski / Wizajny, Suwalki

Following the reference to my friend Debbie Kennet's Cruwys blogspot from 2018, I have contacted her for an update on the relative merits of Ancestry and My Heritage for making DNA comparisons. Here is her reply:

"MyHeritage have updated their matching algorithms since I wrote that post. However, there is still a very high false positive rate because they accept uploads in different file formats and there is very little overlap in the SNPs used for matching on the different chips. See:

 

https://isogg.org/wiki/Autosomal_SNP_comparison_chart

 

I don’t trust matches sharing below about 40 cM at MyHeritage and I’ve also found some problematic matches sharing between 40 and 50 cM. The matches at AncestryDNA are much more reliable.

 

I have two kits at MyHeritage. I did a new test on the Global Screening Array and with that kit 47% of my matches don’t match either of my parents. My other kit is a transfer from Ancestry and with that kit 38% of my matches don’t match either of my parents. I also get over a thousand more matches with the transfer kit.

 

With Jewish matches at MyHeritage I would suggest you ignore all the low and medium confidence matches and only work with matches where the largest segment is 30 cM or more and where are there are preferably at least two reasonable-sized segments. If you look in the chromosome browser you’ll probably find that a lot of the matches where you appear to share high cM totals are actually made up of lots of really tiny segments that are more likely to be false. You may want to recalibrate the total cM shared and remove all the segments under 10 cM to get a more realistic estimate of the total cM shared."

I am aware that a lot of people mistakenly put faith in smaller sized segments when many of these relate to false positives and should be ignored - some of these are the ones joined together to make larger segments by MH. Debbie recommends only looking at those matches where there is a higher 30cm segment plus some larger other segments, and ignore  the smaller ones under 10cm. That is what I normally do anyway, though I have personally looked for one segment of 25cm plus, rather than 30cm, as these seem to be more prevalent.  Though again, I am also aware that you are more likely to get a larger chromosome segment on some chromosomes of around 25cm, which has a skewing effect, so 30cm would avoid that.

This strategy would avoid those matches where clearly no recent family tree relationship can be found, and help in resolving the problem of endogamy. 

Jill Whitehead, Surrey, UK

The dramatic differences between My Heritage and Ancestry are most noticeable when Ancestry thinks the total is <90cM. Above that threshold, TIMBER is not in effect. For Nancy's match to Mary, Ancestry calculates this as 67cM so TIMBER is in effect and they downweight this to 21cM. [Most of the matches you list above are well above 90cM.] Ancestry is basically saying that 2/3 of the DNA shared is too common to be genealogically meaningful. Some folks don't like TIMBER but my experience is that it is very useful, and Ancestry numbers are far more accurate than My Heritage. I doubt this is a traceable match.

The main question is why Ancestry says the total is 67cM and My Heritage says 124cM. If Mary doesn't transfer to GEDmatch, we're left guessing, but given the problems My Heritage has with imputing data, I wouldn't be surprised if that accounts for the bulk of the difference. One researcher who tested both her parents found that 32% of all matches had no match with either of her parents. https://cruwys.blogspot.com/2018/01/a-chromosome-browser-and-new-matching.html That's a lot of bad data!
--
Steve Toub

I decided to try to reframe my earlier post, with the initial responses and new data I've gathered, to place the question in context. 

My original post featured two DNA match reports, one from MyHeritage and another from Ancestry, for my mother Nancy and an unknown person Mary.  MyHeritage reported total shared DNA of 123.8 cM (1.8%), in 10 segments, the largest being 25.2cM (and 3 other segments larger than 10cM).  In turn, Ancestry reported 21cM (66cM unweighted) across 7 segments with the longest 15cM.  I asked the list for help understanding such a wide discrepancy in these reports.  

I receive several responses, here and privately, most sharing a consistent view that Ancestry's algorithms are more conservative while MyHeritage often inflates its reports.  I don't want to go into a lot of detail here (you can see my original post and the full exchange at  https://groups.jewishgen.org/g/main/message/673482) but here is one representative quote: 

Most responses recommended comparing tests at GEDMatch for a more realistic reading.  I should also mention that there were at least a couple of responses warning not to dismiss MyHeritage results, though still acknowledging that their numbers were not as rigorous.

These responses were alarming.  Anyone who has spent any time trying to use DNA to further their research – especially in Ashkenazi inheritance where endogamy muddies the waters – will recognize that a DNA match of 123.8 with a 25.2 segment is considered significant while a match of 21cM (or even 66cM) and a 15cM longest segment is to be given lower priority.  Processing what to do with this information, I wondered whether other matches I'd been exploring also exhibited such variation and were correspondingly questionable.  

I reviewed a table of my mother's DNA matches collected from Ancestry, MyHeritage, GEDMatch and FTDNA (see below).  For the 22 instances where I found matches from two or more testing sites, the average (mean) difference between the highest and lowest reported total was 18cM (when the 102.8 discrepancy reported for Mary was removed from the calculation).  Seven of the 21 (again, excluding Mary) posted above average variations: the largest was 71.6cM and the smallest 1.2cM.  In all but a few cases the variation was not significant and would not affect a determination about which matches to pursue.   

At the same time, a review of the data strongly suggests that assertions that MyHeritage inflates its reports and Ancestry is more conservative don't hold up.  In fact, Ancestry reported a higher total cM than MyHeritage did in 7 of the 9 instances where a head-to-head comparison was available.  And Ancestry posted the highest total cM, compared to any other test site, in half (11 of 22) cases.  

For my part, this leaves me still looking for an answer to my original question:  What can I make of two tests between my mother and Mary reporting such extremely different results?  If I can't explain it away with the algorithms used by the two testing services, what is the explanation?  And, on the practical side, which test do I trust?  I understand that a 3rd party comparison is most likely to answer that question, if Mary is willing to copy her results to GEDMatch.  I've asked her but she hasn't responded so far.  Even if this individual case is resolved eventually what do we make of such discrepancies?  This reminds me of something I found in a fortune cookie: "A person with one watch is certain of the time.  A person with two watches isn't sure."  Testing with more than one service is supposed to help with our family history research, not create more uncertainty.   

Lee David Jaffe
===============
Surnames / Towns:  Jaffe / Suchowola and Bialystok, Poland ; Stein (Sztejnsapir) / Bialystok and Rajgrod ; Roterozen / Rajgrod ; Joroff (Jaroff, Zarov) / Chernigov, Ukraine ; Schwartz (Schwarzstein) / Ternivka, Ukraine ;  Weinblatt / Brooklyn, Perth Amboy, NJ ; Koshkin / Snovsk, Ukraine ; Rappoport / ? ; Braun / Wizajny, Suwalki,  Ludwinowski / Wizajny, Suwalki

A.S.	FTDNA	582
A.S.	MyHeritage	590.4
A.S.	Ancestry	598
A.S.	GEDmatch	603.3
B-D	GEDMatch	99.4
B-D	FTDNA	105
B.F.	FTDNA	62
B.F.	MyHeritage	63.2
B.G.	GEDMatch	118.8
B.G.	FTDNA	130
B.G.	MyHeritage	131.5
B.H.	GEDmatch	57.3
B.H.	MyHeritage	128.9
B.W.	GEDMatch 5	60.1
B.W.	MyHeritage	69.8
D.H.	GEDMatch	175.7
D.H.	Ancestry	205
G.L.	MyHeritage	183.3
G.L.	Ancestry	197
H.R.	GEDMatch	192.9
H.R.	Ancestry	195
J.F.	MyHeritage	85
J.F.	Ancestry	106
J.P.	MyHeritage	138.6
J.P.	Ancestry	148
J.Q.	FTDNA	85
J.Q.	GEDMatch	92.2
J.Q.	MyHeritage	96
M.B	FTDNA	287
M.B	GEDmatch	287
M.B	MyHeritage	300.2
M.L.	GEDMatch	139.2
M.L.	Ancestry	143
M.R.	FTDNA	220.9
M.R.	GEDMatch	221.8
M.R.	MyHeritage	223.5
M.R.	Ancestry	234
M.R.	FTDNA	239
Mary	Ancestry	21
Mary	MyHeritage	123.8
N.T.	FTDNA	125
N.T.	GEDMatch	130.2
N.T.	Ancestry	150
N.T.	MyHeritage	153.5
P.G.	GEDMatch	168.8
P.G.	GEDMatch	172.1
P.G.	GEDMatch	172.2
P.G.	MyHeritage	197.9
P.G.	Ancestry	207
R.A.	GEDMatch	221.4
R.A.	Ancestry	258
R.F.	GEDmatch	315.4
R.F.	Ancestry	324
S.H.	GEDMatch	105.3
S.H.	MyHeritage	115
S.H.	Ancestry	122
S.R.	GEDMatch	104.3
S.R.	Ancestry	118