Considerations in selecting an ad formatThe choice of which ad format to use goes far beyond what seems like it would make the most money. First of all, don't trust advertised eCPM (effective cost-per-mille, or thousand impressions) values -- that's just marketing material, and results will vary, as there are only a few guaranteed eCPM ad campaigns out there. Additionally, eCPM values can't be compared across ad types -- not only can different ad types be displayed at different frequencies, they also have a different effect on the user's perception of the app.
Some ad types (and ad networks) work better with different types of apps or games, and oftentimes the only way to find out is testing. My previous article talks about split-testing to find out which network performs better. A more robust solution is to mediate ad traffic between different networks; I do that with all of my apps that have banner ads, because it allows me to redirect traffic when I discover that one network is not as well suited to a particular app.
Only some kinds of ads (specifically ad types that are common such as banners and other image blocks) are easy to mediate. Specialty ad types usually require the SDK of the hosting ad network, and are not as easily interchangeable with ad formats from other networks. For this reason, I am hesitant to try new ad formats on new networks. That might require releasing an update to my app with the new SDK, and time to add the unique ad format to my app. If I don't like the results, I need to release another update to my app to remove the SDK. Don't let that discourage you from trying new things though -- finding the ad formats that work best for your app can boost revenue from an app significantly.
The decision of which ad format to implement involves a number of factors that can be more than just the advertised eCPM. The ability to easily integrate ads and mediate between networks is of importance to some developers. Perhaps more importantly, the user experience should be carefully considered -- an ad format that fits well with the app can have a positive impact on ad performance.
Tim Mackenzie, author of the Android Income Series books, is a software engineer that escaped the cubicle world at a large company to go solo with Android app development. He uses this freedom to teach others how to make money with Android apps. Visit the ProjectJourneyman.com blog for the information you need to start earning with Android apps.apps.