What are the rotation types?
As a quick reminder pubCenter provides you two ways to filter ad units: by impression or user.

Impression rotation
This rotation type shows various ad units to a web property visitor. With this type of rotation ad units will be served to your web property according to the percent of impressions you assign to each ad unit. For example, you could assign 60% of impressions to go to ad unit A and 40% of impressions to go to ad unit B.

User rotation
This rotation type shows the same ad unit to a web property visitor. With this type of rotation ad units will be served to your web property visitors based on the percentage you assign to each ad unit.

For example, you could assign 60% of users to ad unit A and 40% of users to ad unit B.

What are the differences between the rotation types?
The type of rotation you choose will depend on what you are trying to accomplish.

Impression rotation
If your web property has few visitors, you would want to consider impression based rotation. Impression based rotation will ensure ad units are shown based on a random sampling. This type of rotation will tell you if users respond more to ad unit A or more to ad unit B.

The beauty of this type of rotation is it is easy to understand and is not dependent upon the number of visitors to your web property. The drawback to impression rotation is it is not as predictive as to what will happen, when the test is complete and you are using a single ad unit 100% of the time.

User rotation
If you have a large user base, you would want to consider user based rotation. User based rotation ensures each ad unit is seen and shows a single ad unit to a user. This type of rotation will tell you if a user responds or does not respond to an ad unit.

The benefit of this type of rotation is each user gets a consistent experience, so they don’t realize they are in an experiment (people will perform differently when they know they are being tested) and if the findings of the experiment change over time (bright colors are great the first time, but lose their effectiveness) they are easily detected. The drawback is user based rotation requires a large number of users for statistical significance.

Impression rotation gives you insight into what ad unit performs best.

User rotation tells you how users performed with different ad units.