Proceed with caution when using DNA as part of a genealogical conclusion
Consider tree completeness
When using DNA evidence in evaluating how you are related to an individual, the normal process is to use family trees to find a common ancestor. A study at Ancestry found that only 30% of trees include 3rd great grandparents. Because many trees are incomplete, consideration must be given to the possibility of a relationship in the missing area of trees. This applies both to your tree and the tree of your DNA match. Be cautious when making conclusions using incomplete trees.
Consider confirmation bias
When using DNA evidence as part of a genealogical conclusion, be sure to analyze it critically. If you have a hypothesis about a relationship there is a danger of looking only for evidence that confirms your hypothesis. Always remember to consider other possibilities as part of your analysis. You can minimize confirmation bias by asking others to critique your conclusions.
Consider segment size
Beware of using small segments to formulate conclusions about how you are related to a DNA match. Consider that a significant number of small segments of DNA matching are false. A study by 23&Me determined that 33% of 5cM segments are false. If you match an individual on multiple segments this does not make the validity of small segments more likely. Small segments of DNA might have been inherited from an ancient ancestor or be identical by chance. Matching segments over 10cM in size are likely to be valid.
Adapted from a presentation by Blaine Bettinger at the UGA DNA Night, September 9, 2019
