Sampling in Google Analytics occurs when the program takes a subset of data from your website traffic and reports on trends available in that sample set. Website owners and marketers rely on this data to see how well their website is performing, and to analyze any changes that may have happened to the website.
How is Google Analytics Data Sampled?
When you look at a Standard Report, you will see unsampled data. Meaning, if you look at “All Traffic” during any given period of time, you will see the correct numbers as reported by Google. However, once you add a filter, custom report, or segment to this data (organic traffic, blog traffic, pages with more than 1,000 visits, etc.) your data becomes sampled. You will know your data is sampled when you see the yellow box in the upper right hand side by the date.
By default, this sample number is set by Google at 250,000 visits (not pageviews). However, Google does allow you to adjust this number up to 500,000. If you have a large amount of data, keep in mind this can really slow down the processing time.