Old English had a feminine agent suffix -ster. Webster was a woman who wove, Brewster a woman who brewed, Baxter a woman who baked. Almost all other trade surnames passed down the male line — these did not.
Websternot a male weaver — a woman who wove
Brewstera woman who brewed
Baxtera woman who baked
How this is counted. The rule is the suffix: a surname counts only if it ends in the Old English feminine agent -ster. No judgement, no interpretation — the string either ends that way or it does not.
The frequency here — 2% — is measured against
159 surnames, which is our corpus and not the English language.
Run the same rule over a different corpus and the number will differ; what will not
differ is which surnames carry the marker, because that is decided by the string,
not by the sample.