Definition
A measurement methodology is the fixed set of choices behind a visibility number: which engines, which question set, whether web search is on, and what counts as a mention. Changing any of them changes the ruler, not the thing being measured.
Why it matters
In one real case, enabling web search moved a site's mention rate from 3% to 13% while the site itself did nothing. Presented as a trend, that is an internal change sold to a customer as their own improvement.
How to measure it
Stamp every stored run with a methodology version and refuse to compare runs across versions. Show the version in the interface so a break in the line has a visible reason.