"YDNA Signatures" are a composite set of YDNA values developed from multiple persons, with an emphasis on identifying the "common denominator" that best characterizes the group considered. Typically, a signature may be shared by decendants of persons for whom there is no immediately obvious relationship, and which may trace their ancestry to persons from widely separated geographic areas. Data for persons who believe they are descended from a common ancestor for a particular signature, but whose YDNA values differ from the signature by more than 10%, are not included in defining the signature. Generally speaking, persons with a set of YDNA results that differ from a Signature by more than about 10% would not be expected to share a common ancestor in a reasonable genealogical timeframe. The accuracy of the above statement varies considerably with the number of markers included in a test result. Testing a large number of markers would tend to give a more robust result. That is, a 12 marker test does not give robust results to begin with, and adding additional markers to a specific 12 marker test can considerably change how it corresponds to any given signature.
A somewhat aribtrary geographic by-name, such as the "Pequea Creek Cowan Signature", is used to describe the signature in discussions. The use of the signature for a particular line is not intended to imply that the line arose from the geographic area denoted by the signature byname. For example, the signature applied to a person descended from someone who settled in South Carolina might share the same YDNA signature as descendants of someone who settled on Pequea Creek, and both would be characterized as having the "Pequea Creek Signature".
Develpment of a YDNA signature is based on the examination of numerous YDNA test results for persons bearing the surname of interest. These data are readily available through sites such as Family Tree DNA, and Y-Search. To determine a signature YDNA data is organized tabularly, and a "target" lineage is selected. Each test result is then compared to the target in terms of the number of "deviations" between the two. The total number of deviations between each test result and the target are recorded for the test result, and the "percent deviation" is calculated. Data for the test results that show 10% or less deviation from the target are then compared. to determine the most common pattern across those results. For each marker tested, the most common result is selected, and becomes part of the signature. The final signature is commonly similar to, but not exactly the same as the original target.
Persons are considered to possess this signature if their YDNA profile includes fewer than 10% "deviations" from the signature. Percent deviation has been chosen as the critiera because if allows comparison between profiles based on many markers (e.g., 64) and few markers (e.g., 12). There is, however, a difference in reliability between tests with few and many markers. Reliability is considered to be quite high for 36 or more marker tests, and relatively low for 12 marker tests. For this reason some 12 marker test results, even though they match the criteria (in this case 1 or fewer mismatches), probably do not truely share the signature, and as a result do not descend from a relatively recent common ancestor compared to others shown here.