OK, that could be one of the worst blog titles I’ve written, but it’s intentional. When people share this post, I want the title to clearly convey the lesson.
Small Segments are Poison
We know that many small segments are false, and thus that many distant matches are false positives. I have written about small segments and distant matches many times. For a few background articles, see the following:
- “Small Matching Segments – Friend or Foe?“
- “The Danger of Distant Matches“
- “The Effect of Phasing on Reducing False Distant Matches (Or, Phasing a Parent Using GEDmatch)“
The (most current as of September 2017) definitive article on the nature of false versus true small segments is “Reducing Pervasive False-Positive Identical-by-Descent Segments Detected by Large-Scale Pedigree Analysis.” The paper is available online for free (http://mbe.oxfordjournals.org/content/31/8/2212). In the paper, the researchers found that more than 67% of all reported segments shorter than 4 cM are false-positive segments. At least 60% of 4cM segments were false-positive, and at least 33% of 5 cM segments were false-positive. The number of false-positives decreased fairly rapidly above 5 cM. See my analysis of this paper here.