Your AncestryDNA® test results are the product of a multi-step process. First, your DNA sample is measured, or read, in the lab. This lab processing generates raw DNA data. The raw data is then analyzed to generate your AncestryDNA® results. There is no single measure of AncestryDNA® test accuracy. Instead, the accuracy and precision of each step can be measured independently.
Accuracy of the Reading of the DNA
Reading your DNA is a first step in generating your AncestryDNA® results. Accuracy is very high when it comes to reading each of the hundreds of thousands of positions (or markers) in your DNA. With current technology, AncestryDNA® has, on average, an accuracy rate of over 99 percent for each marker tested.
Precision of Your Ethnicity Estimate
When you take an AncestryDNA® test, part of your genetic testing results will include an ethnicity estimate. Part of this is an estimate—reported as a percentage—of how much of your DNA looks like the DNA of people in different parts of the world. It can reflect the family history where your ancestors lived hundreds of years ago, and even as far back as 1,000 years ago. An example would be a result of “8% Northern Italy,” which reflects the amount of your DNA that looks like the DNA of people who have lived in Northern Italy for many generations.
AncestryDNA® determines this part of your ethnicity estimate in two steps. The first step is to collect the DNA of people whose family has a long history in a particular part of the world. This group is called the reference panel, and as of 2023 over 80 different regions in the world are represented.
The second part of the process is to compare your DNA, bit by bit, to the DNA of the people from the more than 80 different regions in the reference panel, to see which groups’ DNA your DNA most resembles. For example, if ten percent of your DNA looks most similar to the DNA of people from France, AncestryDNA® will assign ten percent of your ethnicity estimate to France.
How Precision is Determined
AncestryDNA® uses a number of different methods to determine the precision of our ethnicity estimates. One method looks at how well AncestryDNA® predicts the ethnicity estimate of people with a known ethnicity. For example, it looks at how well it works on people from the reference panel (who should theoretically come back with 100 percent of a certain ethnicity).
The precision of particular ethnicity regions in the estimate depends upon the specific region/population and the granularity of the prediction. See the Ethnicity Estimate White Paper for more information on how well the AncestryDNA® algorithm does for each region of the reference panel.
When it comes to your own results, the percentages you see for your ethnicity regions are the most likely results we get after comparing your DNA to our reference panel. But there are also a range of percentages that are nearly as likely. This is why we present your results as a range of possible percentages. For example, if you see “41% France” in your AncestryDNA results, the actual range of most likely results might go from 38-45% France. In other words, between 38%-45% of your DNA looks like the DNA of people in our France reference panel, with the most likely percentage being 41%. You can find this range by clicking on a specific ethnicity within your AncestryDNA® report.
Accuracy in DNA Matching
DNA matching is an opt-in feature of AncestryDNA® test results. With this feature, we identify people you may be biologically related to and how you may be related to them. For example, someone else who has taken the AncestryDNA® test may be identified as a possible 2nd to 3rd cousin. These results can provide fresh insights into your family tree by helping you to identify other biologically related family members.
An AncestryDNA® test can very accurately determine if two people are genetically related at the 3rd or 4th cousin and closer level. This is because AncestryDNA® test results are based on the scientific method that identifies people who share long segments of DNA. Usually this is only possible if two people have had a recent common biological ancestor.
The AncestryDNA® test increases the accuracy of DNA matching by using methods to rule out cases where unrelated people happen to share a stretch of identical DNA. One way we do this is by limiting the size of the shared DNA. AncestryDNA® also uses a special algorithm called TIMBER to filter out any larger pieces of identical DNA that may be shared for a reason other than a recent common ancestor. With this methodology, we can be extremely confident in finding people you are genetically related to.
Challenges in Predicting Exact Relationships in DNA Matches
Where things get a little harder is predicting the exact level of genetic relationship between matches. AncestryDNA® can assign a parent/child or a sibling relationship with a very high degree of confidence. Other relationships are more difficult to assign for biological reasons.
Some relationships are difficult to assign because many people happen to share the same amount of DNA. For example, a grandparent and grandchild, a double first cousin, a half-sibling, or an aunt or uncle all share around 25 percent of their DNA with each other. This obviously makes them hard to tell apart DNA-wise. So AncestryDNA® reports back a “Close Family” relationship instead of one of these specific relationships. It is up to you, the customer, to use other methods to determine which of these relationships is correct.
Other relationships are difficult to assign because the amount of DNA they share can overlap. For example, because of how DNA is passed down, sometimes you and your second cousin might share as much DNA as someone else and their first or third cousin. And there is more overlap the more distant the relationship: it is not easy to tell a 3rd cousin from a 5th cousin, for example.
Ongoing Commitment to Accuracy and Precision
The kind of science done at AncestryDNA® is cutting-edge. This means that as science advances, the AncestryDNA® results will continue to better reflect the history of where your ancestors lived and who you are related to. In addition, as more customers from more parts of the world participate in the AncestryDNA® test , both the ethnicity estimate and matching results will also get better and better.
