Set up a share-of-voice order
Let's say you want to distribute impressions evenly among several advertisers for a set period of time. For example, you've sold the 300x250 on your homepage to five different advertisers over the same period of time, and you'd like the advertisers to rotate evenly. This approach is often called "even share of voice." Some publishers choose to sell their inventory as a simple balanced rotation rather than on a reservation basis with impression goals.
You can accomplish even delivery distribution by setting the delivery priority of multiple overlapping line items to 'Sponsorship'. This method works because sponsorship line items are served on a percentage basis rather than on an impression goal basis, so sponsorships with the same percentage goals should be delivered the same amount. Also, if the line items have multiple creatives, set the Display creatives option to 'All', 'One or more' or 'As many as possible'.
To set up a share-of-voice order:
Set up an order.
Create a line item for Advertiser #1.
In the line item, set the delivery priority to 'Sponsorship' and the delivery goal to [100 ÷ Number of Advertisers].
Finish setting the targeting options for the line item and Click Save.
Repeat Steps 2-4 for all overlapping advertisers by creating corresponding line items under the same order.