To get an overview of the performance of your email marketing efforts, you can compare any or all of your sent campaign reports.
To see how clicks, opens and other email activity are trending, open Campaigns to find your list of "Sent Campaign Reports." Select the checkboxes next to two or more reports then click Compare selected campaigns, as shown here:
To compare all of your sent campaign reports, select the checkbox positioned on the top-left of the table. You can compare up to 100 sent campaigns, but you'll likely get the most value by comparing a smaller set.
For example, if you've made changes to your campaign copy, template design, or the way you collect new subscribers, you can see if those changes have made a difference. Here's an example of a comparison report page:
Use the Export button at the bottom of this page to download your comparision report as a CSV file, or tab delimited text file.