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How Segments are calculated

If your site has more than 10,000 daily page views, then everything beyond the most recent 10,000 page views will be sampled, up to 100,000 total.

The vast majority of sites have less than 10,000 daily page views so it is likely that the data you see will be 100% accurate. Sampling may come into play however for multi-day reports, as the absolute max sample size for any single report is 100,000 page views. So if you were viewing a report for a month with 30 days for example, then the max sample size for each individual day would be 100,000 / 30, or 3,333.

Because of sampling, you may occasionally see some rows without Segments data.

The default sampling rate beyond the last 10,000 page views is 25%, but you can change the sample rate in your user preferences. There are 4 options available ranging from 25% to 100%, as well an option to disable this feature (if you don't want it or want the reports to load faster), but again, 100,000 is the maximum page views that will ever be sampled for a single report.

Pro Plus or higher is required to use a sampling rate higher than 25%. Even if you are maxing out a standard Pro account, which allows up to 30,000 daily page views max (as of April 2015), your overall sample rate would still be 50%. This is because the last 10,000 page views will always be used, and beyond that, 25% of the remaining 20,000 is 5,000, for an overall sample size of 15,000 out of 30,000 page views, or 50%.