A drop in visits might just be summer. A drop in orders never is. Stats measures your audience without cookies — and above all the events only your server knows about: a sign-up, a payment, an export. Then it compares a Tuesday to recent Tuesdays, and tells you when it collapses.
A visitor is recognised for the day by a fingerprint computed with a salt drawn at random each night, then thrown away. The next day, the same person is someone else — irreversibly. Nothing to declare, nothing to make anyone click.
One POST /api/v1/evenements from your server: “order_paid”, 49.90. That is where a collapse shows, and no tag inside the page will ever see it.
Comparing to yesterday would set off an alert every Saturday. We compare to the same weekday over previous weeks, on the last complete day — and we stay quiet while the history is too short.
Visitors, visits, page views, bounce, time on site — each against the previous period. Pages, entries, exits, sources, campaigns, countries, devices, browsers, systems, languages.
Not the IP address, not the user-agent, not the full referrer URL — a URL can carry a token. Only labels shared by millions of visitors: “Firefox”, “Linux”, “en”.
Your sign-ups drop on 3 March; Watch saw your SPF record change on the 2nd. No analytics tool can say that: it does not look at your email. Here, it is all one house.