Currently on per-domain uniques, fresh sessions computation named offset counts fingerprinted sessions having made:
- 1 request with no cookies set (nocookie IS NOT NULL)
- 0 request with some cookies set (nocookie IS NULL).
This way of computing the offset undercounts the fresh sessions.
While making sure we count only devices having made 1 request with no cookies set (nocookie IS NOT NULL) is correct, restricting the number by counting only devices having made 0 other request prevents counting devices whose "fresh" session includes more than 1 hit, about 10% of the offset.
Fresh sessions with more than 1 hit will have a first hit with nocookies=1 and that will set the cookie of that day, say, if current date is May 3rd it will set last-access cookie to May 3rd. The subsequent requests from that session have last access set to May 3rd and thus, are not counted towards uniques of that day (only requests whose date is less than current date get counted)
Move to production:
- Add a row in documentation about the change in this page