OpenX Tips

In OpenX, it is possible to apply capping to banners. Capping allows you to set a maximum number of times a banner will be seen by a user. This limitation can apply:

• For the browser session — that is, the limitation on the number of times a user will see the banner will apply until the user closes their web browser, at which time the capping limitation count will be re-set; or
• For all time — that is, once the user has seen the banner the number of times specified by the limitation cap, they will not see the banner again, even if they close and restart their browser.

With either type of capping, you can specify an optional time period. After this time period, the cap will be re-set, and the user will once again be able to see the banner, until the cap is reached again.

In addition to setting capping at the banner level, capping can also be applied at the campaign level. In this case, the capping is applied to the aggregate impressions for all banners in the campaign, and once the limit is reached, no banners from the campaign will be shown.

As an example, consider the banner shown below. It has been set up with a capping level of 2 impressions for all time, with a capping re-set period of 24 hours. You have already seen the banner once now, so, if you reload this page, you will see the banner for a second time — which is the limit. Should you re-load the page a second time, you will not see the banner, as you have already reached the limit of two impressions. (But if you come back tomorrow, the banner will once again show for another two impressions.)

The delivery capping options set for the above example banner.

With regards to the above example banner, no guaranteed delivery banner has been included in the zone being displayed, which is why no banner appears once you have reached the 2 impression limit per 24 hours. This highlights why, in a real-world OpenX setup, you should always have a guaranteed delivery banner in every zone.

Why use capping?

Capping banners is not for everyone. Indeed, it makes little sense to apply capping to banners that are there to simply take up unsold inventory.

However, it is widely believed that as the number of times a user has seen a banner increases, the likelihood of that user clicking on the banner decreases¹. As a result, if you are running CPC or CPA campaigns, you may find that you will make better use of your inventory by applying capping, so that you don’t show the banners to these users too many times — and once the capping limits have been reached for a given user, then you can show the user the lower earning CPM campaigns.

Cookies & first time users

One important thing to note about banners with delivery caps applied is that they are managed using cookies. If OpenX cannot set cookies, then banners with delivery capping applied will never be shown. This is because, if capped banners were shown to users who had cookies disabled, then these users would be able to see capped campaigns infinitely. For cases where you have actually sold inventory on a capped basis, this would obviously be very bad — which is why OpenX does not do it.

However, this does mean that on the very first visit a user ever makes to your site, OpenX will not yet know if it can set a cookie or not. As a result, on this very first ever impression, OpenX will not deliver a capped banner. It will wait until the second impression to do so, when it can then be sure if it was able to set a cookie from the first impression. This may be important, if you absolutely require that a certain banner be shown to all users on their first visit to your site — in this case, you will need to use an uncapped banner.

1. It is surprisingly hard to find any actual evidence to back up this belief, though. Please comment below if you have a reference to any! [↩]