Know what your numbers mean.
What each metric counts, and which one is an estimate.
Visitors — the only estimate here
A distinct count of the daily visitor hash. Because the salt behind that hash rotates every 24 hours, the count is per-day: a multi-day figure sums daily distinct counts rather than deduplicating across them, so someone visiting on Monday and Thursday counts twice in a weekly total. Someone switching network or browser also counts twice. This is the cost of having no durable identifier, and no setting changes it.
Visits and pageviews — exact
Visits are stitched sessions and pageviews are recorded page loads. Both are counted, not estimated. When you need a number you can defend, use these rather than visitors.
Bounce rate
The share of visits that did not continue past a single interaction. An event sent with interactive set to false — a heartbeat, say — deliberately does not rescue a bounce, so background pings cannot flatter the number.
Visit duration and engaged time
Visit duration is measured between the first and last recorded activity in a visit. Engaged time is separate: it comes from the engagement pings and measures active time rather than elapsed time, which is why a tab left open in the background does not accumulate it. Single-page visits have no second timestamp, so duration is weakest exactly where engaged time is most useful.
What you can break any of them down by
Page, entry and exit page, hostname, source, channel, referrer, the full UTM set, device, browser and version, OS and version, country, region, city, event name, and any custom property you attach.
When today’s numbers settle
Reports read daily aggregates, rebuilt every 15 minutes, so figures for today fill in as the day progresses. A day settles once the following day is complete, because a visit that runs past midnight belongs to the day it started. Re-running a rollup repairs a day rather than double-counting it.