What is the difference between dimensions and metrics, and what is data sampling and thresholding?

Dimensions are attributes; metrics are numbers. A dimension describes what — country, device category, page path, source, medium. A metric measures how much — users, sessions, conversions, revenue. Every report is metrics broken down by dimensions.

Custom dimensions and metrics let you register your own event or user parameters so they appear in reports. Event-scoped for something varying per event, user-scoped for a persistent attribute such as membership tier. Registering them is required — an unregistered parameter is collected but not reportable.

Data sampling means GA4 analyses a subset and extrapolates when a query exceeds a processing limit. Standard reports are unsampled; Explorations can be sampled on large properties or long date ranges. GA4 shows a sampling indicator, and you should check it before drawing conclusions — a sampled report on a small segment can be badly imprecise.

Data thresholding is different and catches people out. GA4 withholds rows when they could identify individuals — typically when Google signals is enabled and the underlying user count is small. The row simply disappears rather than showing zero, so totals appear not to add up.

Note: The practical answers: widen the date range or narrow the query to reduce sampling; and to avoid thresholding, either disable Google signals in reporting identity or use the free BigQuery export, which contains the raw unsampled, unthresholded event data. Naming BigQuery as the escape hatch is the answer that demonstrates real depth.