Quality
Ad blocker
Also called content blocker.
Ad blocker is client software that blocks known tracking hosts or script patterns. Some of it stops analytics scripts too, not only ads.
How it is measured
The blocker holds filter lists of hostnames and URL patterns. When a page asks for a matching script or beacon, the request is cancelled before it leaves the browser. Your server never sees it, so no log, count or error exists.
Tarsier is served from your own deployment, not a shared analytics domain, so common third-party lists are less likely to match it. Some lists still block by path or script name, and a blocked visit simply goes missing.
Worked example
A developer-tools site compares its analytics with the CDN access log for one Tuesday. The CDN shows 11,400 page requests from real browsers and the tracker shows 8,700 pageviews. About 24 percent of the gap sits with visitors who use content blockers, which is high because the audience is technical.
A lifestyle blog on the same stack shows a 7 percent gap.
How it differs
An ad blocker hides visits from client-side analytics. Tracking prevention is the browser's built-in version. A discrepancy is the visible gap between counts. A JavaScript tag is what the blocker targets.
Common errors
Assuming everyone is counted. Blaming bots for a gap that blockers explain. Serving the script from a famous shared host. Disguising the script to dodge consent. Ignoring that technical audiences block more.
In practice
Compare your tracker with server logs for one day a quarter to estimate the blocked share. Quote analytics as a floor, not a headcount.