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Data Sampling in Google Analytics - What Is it and Why Google Uses it?

Sampling is the process of selecting and processing a sample of the whole data in order to get faster results. Google Analytics uses sampling for large sets of data and many of us aren't even aware of this. Data sampling is necessary because it helps to increase the speed with which Google processes the data. When sampling happens in a report, a message gets displayed as "This report is based on N sessions", where N is the total number of sessions involved in the sampling process.

When Does Sampling Occurs?

Sampling occurs automatically when more than 500,000 sessions are collected for a report. Under Multi-Channel Funnel reports, Flow Visualization reports are sampled after 100,000 sessions. So, generally sampling occurs when large sets of data are involved. 

Can We Adjust or Stop Sampling?

Yes, you can stop sampling if you wish. Sampling leads to increased processing speed with a minor loss in accuracy so if you wish to stop sampling then data accuracy will surely improve but with a loss is processing speed. 

While viewing a sampled report, you have the option to adjust the sampling rate. You may adjust it in order to increase accuracy and affect how quickly the report loads. To adjust the sampling rate, Click the dropdown menu at the end of the menu bar. Select the preferred option from the available choice of options. The data will be automatically refreshed using the new setting. if you wish, you may change this setting.

Premium Google Analytics users have the option to view unsampled reports for some sets of data. However, there are certain limitations, the number of data rows to export cannot exceed 3 million and the number of sessions per property cannot exceed 200 million.

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