Sampling
Also called data sampling, sampled data.
Building a report from a portion of the data and scaling the result up to estimate the whole.
When a report would need more data than a tool processes per query, it can read a portion and scale up. The result is an estimate, close for big numbers and rough for small segments.
In GA4, sampling can happen when a report, exploration or API request needs more events than the property's quota: 10 million events per query for standard properties, and up to 1 billion for Analytics 360. The data quality icon shows the share of data used (Google). Standard reports are rarely sampled; explorations over long date ranges are where it shows up.
Other tools sample at different points. Sporetag stores events in Cloudflare's Workers Analytics Engine, which samples at high volume and records the weight of each kept event, so totals are scaled back up when read (Cloudflare).
To reduce sampling, shorten the date range, or use a report that reads from pre-aggregated tables.
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