Ancestry will compare your DNA to the DNA of one of your matches, then calculate a confidence score for you. This score lets you know how much DNA evidence there is for you and your match actually being related.
Just because you and another member have identical DNA doesn’t mean that you both inherited that DNA from a recent genealogical ancestor. That’s where the confidence score comes in, when they calculate the likelihood that you and your DNA match are actually related. A high confidence score means that they are almost positive that your DNA is identical because you have a recent ancestor. A lower score means that your identical DNA might be because you’re related, but it might also be because you have similar ethnic or regional backgrounds.
Now Ancestry also shows the shared centimorgans (cM), which is a unit used to measure the length of DNA. The higher the number, the higher the confidence, and in general, the closer the relationship. The number of segments and number of centimorgans that they show reflects only those segments they believe were inherited from a recent common ancestor. This is the reason that the number of matching segments and cM is commonly different on Ancestry than on other sites. It isn’t counting some segments that other algorithm’s will count.
Ancestry has a white paper available for download on their site that has the algorithm that it uses. From the white paper is this paragraph “Timber allows us to measure the evidence that each matched DNA segment is due to recent common ancestry. By taking advantage of our very large database of sample genotypes, we are able to use this evidence to, in an individual-specific manner, rescale each match based on the likelihood it is actually IBD. Through rigorous testing, we have found that Timber eliminates a large majority of false positive IBD segments. As we demonstrate in the following section, this significantly reduces the number of false positive relationships estimated between pairs of individuals in practice.” In other words, the Timber algorithm is smart enough to not count segments other software does count. Ancestry is also using larger minimum thresholds than other sites.
From DNA Explained.com “Timber is removing 64 to 87 VALID segments in parent/child matching, believing that pileups are invalid. Rule #1 of DNA – you must match your parents. If you double this number, because you have two parents, each person has in the ballpark of from 130 to about 200 areas where their DNA is “too matchy” and segments/matches are removed. This illustrates the magnitude of the Timber problem.
You cannot draw or correlate any relationship inferences from either the total amount of shared DNA nor the number of segments by utilizing the typical tools utilized by genetic genealogists because Ancestry’s totals will be lower and their segments will be broken into more pieces due to the removal of segments identified by Timber as invalid matches.”
Ancestry continues to make advances. They still need a Chromosome browser and there tends to be hiccups with sending people messages and leaves stalling out on trees when you move the DNA around, but in general it continues to improve. Just be aware when you are comparing these new cM numbers to relationship charts that the numbers may not be reliable for prediction in the same way. Download your raw DNA (DNA page – settings wheel- download Raw DNA) and upload it to Family Tree DNA and use their Chromosome browser to look at your shared segments.