In standard Google analytics properties, a yellow shield icon at the top of reports we are viewing indicates sampling in effect. We experience sampling when we query ad-hoc data more than the expected threshold. For standard properties the threshold is at 500k and for 360 users it is set at 100M. This Google doc indicates different sampling limits for MCF , flow visualisation and ad-hoc reports.
For sampled reports in standard Google analytics property we are given two options to choose suitable sample size i.e. greater precision and faster response. Greater precision yields maximum sample size whereas faster response yields smaller sample size. This feature is not available for App+web properties. The reason for this could be that standard reports are always unsampled even if we apply secondary dimensions,filters or choose wide date range.
Though sampling doesn’t exist in standard reports they do exist when we build reports using advanced analysis. Any non-pre-existing standard report build using advanced analysis which exceeds 10 million events/hits experience sampling.