What is Data Sampling?
In Google Analytics it means selecting a subset of data from your website traffic.
Why is this done?
The idea is that using a subset of data will provide comparable results to using the full amount of data available. Using a smaller data set will speed up the process for reporting, as pulling larger amounts data slows down queries.
When will I see Sampled Data?
When a report collects data from a large data set, over 500,000 visits, visitors or pages, you will see that the data collected is sampled. While running Multi-Channel Funnel reports, sampled data will be used when you have over 1 million conversion paths.
When running reports in Google Analytics you may see a yellow box at the top of the report which says:
This gives you specifics on the percentage of visits that the report samples from. As you can see, the average of visits is a little over 210,000, but the percentage of visits is lower in each instance, based on the amount of data each report has to sample from. The larger the amount of data pulled, the lower the percentage of visits that will be sampled into the report.
A new feature in Google Analytics is the Adjust Sample Size tool. The slider, which is located below the date range, allows the user to choose between faster processing and higher precision.
This tool will allow you to adjust the sample size from the default of 250,000 (which is the center of the slider) up to 500,000 visits. As you will see in the samples below, the data samples differ depending on where the slider is placed. The c can be placed anywhere across the path, not just on either end or the middle of the tool.
When you choose a sampling threshold, that preference will be used in all reports until you close Google Analytics.