To Compare Campaign Stats or Aggregate Campaign Stats, navigate to the Email Campaign Statistics page and select the campaigns you wish to run stats on (Fig 1).

Fig 1 

After selecting the campaigns you want to run reports on, on the drop-down menu select either Compare Selected Campaigns or Aggregate Selected Campaigns, then click the Go button (Fig 2).

Fig 2 

You'll see a modal asking if you want to exclude some sends; make your appropriate selection to advance to the appropriate report (Fig 3). 

Fig 3

If you exclude enough sends where your campaigns have no data, you might hit an error that looks similar to this (Fig 4). 

Fig 4

Aggregating Campaigns shows a report that should look something like this (Fig 5).

Fig 5 

Comparing Campaigns shows a report that should look something like this (Fig 6).

Fig 6