Author Topic: Centimorgans and Segments  (Read 728 times)

Offline Jill Eaton

  • RootsChat Senior
  • ****
  • Posts: 325
  • Census information Crown Copyright, from www.nationalarchives.gov.uk
    • View Profile
Centimorgans and Segments
« on: Wednesday 24 January 18 13:05 GMT (UK) »
Would any experienced DNA experts/Semi-experts/or people with more knowledge than me (I'm a novice) be able to give me a realistic estimate of how many centimorgans and/or segments would count as genuine match worth chasing?

I'm sceptical that the Ancestry relationship estimate is accurate. Sometimes it seems a little over-enthusiastic.
Davis - Berkshire & London
Sutcliffe - Yorkshire & London
Harrington - Ireland and London
Fuller - Cambridgeshire and Essex
Waldron/Waldren - Devon & London
Frisby and Lee - Leicestershire
Hollingsworth - Essex
Williams - Ireland? and London
Ellis, Reed & Temple - London
Lane - ?
Surplice - Cambridgeshire
Elwood - Cambridgeshire

RootsChat is the busiest, largest free family history forum site in the country. It is completely free to use. Register now.
Also register instantly with Facebook or Twitter (and other social networks). Start your genealogy search now.


Offline rsel

  • RootsChat Senior
  • ****
  • Posts: 258
  • Census information Crown Copyright, from www.nationalarchives.gov.uk
    • View Profile
Re: Centimorgans and Segments
« Reply #1 on: Wednesday 24 January 18 16:54 GMT (UK) »
Would any experienced DNA experts/Semi-experts/or people with more knowledge than me (I'm a novice) be able to give me a realistic estimate of how many centimorgans and/or segments would count as genuine match worth chasing?

I'm sceptical that the Ancestry relationship estimate is accurate. Sometimes it seems a little over-enthusiastic.
I am no expert, but GedMatch talks about 7cm or larger as the basic for a good match

Richard
Sellens - Sussex
Newham - Surrey
Wellington - Dagenham, Essex
Camp - South Essex
Wren - Essex
Livermore - Essex
Wane - Essex
Fisk - Essex / Suffolk
Bailey/Bayley - Sussex
Newton - Sussex
Funnell - Sussex
Streeter - Sussex
Coates - Sussex
Maisey - Surrey

RootsChat is the busiest, largest free family history forum site in the country. It is completely free to use. Register now.
Also register instantly with Facebook or Twitter (and other social networks). Start your genealogy search now.


Offline Eric Hatfield

  • RootsChat Member
  • ***
  • Posts: 174
  • Sydney, Australia
    • View Profile
Re: Centimorgans and Segments
« Reply #2 on: Tuesday 30 January 18 06:29 GMT (UK) »
Yes, the cutoffs used by the major companies and webpages are a good guide, and several places use about 7 cM as their cutoff. This reference gives some details .

There are two issues in this - false positives and false negatives. The more you eliminate one of these by where the cutoff is set, the more likelihood there is of the other.

False positives:

This table shows that 7 cM matches are 50% likely to be genuine and 50% likely to be by chance, while at 10 cM 90% of matches will be genuine. I am a little surprised at those numbers.

False negatives:

This table shows how likely the three main testing companies are to find (or miss) a match. First and second cousins will almost never be missed, 90% of third cousins will be identified, but only around 50% of fourth cousins and very few 5th cousins will be identified. Of course if they set their criteria lower, they would pick up a greater percentage, but also pick up more false positives. I think they have the balance about right - they try to miss few closer relatives even though this generates a few more false positives.

One answer to this is to have several relatives test. I didn't match using FTDNA or Gedmatch criteria someone who is actually a third cousin, but my cousin and aunt (who were third cousin and second cousin once removed) did match and so conformed the relationship.