What the calculator actually computes
A G25 admixture calculator takes your 25 coordinates and a list of source populations, and searches for the percentages that make the weighted average of those sources land as close to you as possible. The percentages cannot be negative and must add up to 100.
The fit distance is what is left over: the straight line distance, across all 25 components, between your coordinates and the best mix. Zero would be a perfect reconstruction. Every result should be read together with its distance, because a model with a poor distance is reporting percentages of the wrong ingredients.
What counts as a good fit distance
The MYDNATLAS PDF report grades each fit on this scale:
| Fit distance | Grade |
|---|---|
| below 0.015 | Elite fit |
| 0.015 up to 0.025 | Excellent |
| 0.025 up to 0.04 | Good |
| 0.04 and above | Fair |
Treat the scale as a guide, not a verdict. Some ancestries have close references and fit tightly; others, such as populations that were never sampled in ancient times, will always sit further from every model. What matters more is the gap between your best model and the next ones.
Why a low distance is not proof
- Near ties. Different source sets can reach almost the same distance. When they do, the data cannot tell them apart, and any single percentage is less certain than it looks.
- Proxies. Sources are sampled individuals or population averages standing in for the real ancestral groups. A source that fits well may be a cousin population of your actual ancestors.
- Extra sources. Adding a source can only lower the distance or leave it unchanged, so a five way model will usually beat a three way model by a little. Prefer the simpler model when the difference is small.
- Input quality. Coordinates estimated from a consumer chip depend on how many of its SNPs overlap the reference panel. A noisy point gives every model a noisy answer.
How to check a result yourself
- Look at the top several models in Multi-Way Fits, not just the first. If the same sources keep appearing, the signal is real.
- Check where you sit on a PCA plot. A model that places you between its sources should agree with what the plot shows.
- Read the closest populations list. Sources that are nowhere near your nearest references deserve suspicion.
- Make sure the sources are historically possible for the period you are modelling.
- Rebuild the model by hand in Vahaduo with the Vahaduo guide if you want a second opinion on the same coordinates.
Getting started
Every check above needs your G25 coordinates. The raw DNA to G25 guide shows how to get them from 23andMe, AncestryDNA, MyHeritage or FamilyTreeDNA.
Questions
How accurate is an admixture calculator?
As accurate as its reference samples and as honest as its reading. The arithmetic is exact: the calculator finds the mix of sources that sits closest to your coordinates. What it cannot do is prove that those sources are your real ancestors, because other source sets can fit almost as well and the true ancestral populations may never have been sampled.
What is a good G25 fit distance?
There is no universal cut off, because the distance depends on the sources available for your ancestry. As a working scale, the MYDNATLAS PDF report puts a fit below 0.015 in its top grade (Elite fit) and a fit of 0.04 or more in its lowest (Fair). Compare models with each other rather than against a single number.
Why do different calculators give me different percentages?
They use different reference populations, different source choices and different methods. Testing company ethnicity estimates are built on each company's own reference panel and are not G25 models, so their percentages are not comparable with a G25 fit.
Does adding more sources make a model more accurate?
It makes the distance lower or leaves it the same, because a source the fit does not need can be given zero weight. A lower distance from extra sources is not evidence on its own. Keep the simplest model whose distance is close to the best one.
Is the analysis free?
No. Multi-Way Fits and the other analyses run on your own DNA and are part of the paid plans. The Population Encyclopedia is free to browse with a free account.