From the Reviews Tab, use the drop-down menu and select "Store Comparison". Select a pre-filled date range or enter your own. The report defaults to the last 3 months. If you click the box to "Compare with Previous Period", it will show percentage changes (red for a decline, green for an increase) for the previous period of the same range (example: Store Comparison Report - last 3 months vs. previous 3 months). You can also view the comparison by a group when you click "Show Groups".
After you have selected the range, providers, and comparisons you wish, you can download the report into .xls / .csv file.