Data for a report is sampled if it is based on more than 500K visits. When you view a sampled report, for example, when you have used Advanced Segments to analyse different types of traffic, you have the option to adjust the sampling size in order to increase accuracy and affect how quickly the report loads.

The sampling icon appears at the right end of the menu bar below the dates that the data is captured.

When you select the icon a box appears with a slider. Faster Processing part of the slider offers the highest sampling level where the lowest number of visits is used in calculation. The Higher Precision part of the slider offers the highest number visits available for this report after sampling.

Adjust the slider depending on what you want. Once you release the slider, the report data is automatically refreshed with the new sample. This setting is remembered across all reports until you close GA.