When viewing Site Trends, the Comparison Metric allows you to see how the performance of a particular Site Trend KPI (in the current timeframe) compares to the same period in the past.
Example: If Site Trends shows daily data, you can see how the KPI performed on the previous day or the same day last year. If Site Trends shows weekly data, you can see how the KPI performed during the previous week or during the same week last year.
The comparison option is available on both the summary and the detail view. Note that you can only change the period reported on in the detailed view.
If the view shows daily data, you can select either Same Day Last Year or Previous Day. If the view shows weekly data, the Comparison Metric options change to Same Week Last Year or Previous Week.
The comparison periods (daily, weekly, monthly, and quarterly) are determined by the Period you selected in the detailed view. Until you change the period in the detailed view, the summary view will show daily data. When you change the period in the detailed view, the summary view will show data for that period until you change the period again.