When the Yahoo! Publisher Network came out of beta, many content publishers saw an opportunity to increase their contextual ad revenue. The problem was, many publishers that were already having great success with Google AdSense were reluctant to start from scratch with a new program having no idea what type of revenue they might be looking at. That's where testing comes in handy.

Since each of the engines require that you run their ads exclusively on a page, the only way to do true testing is to setup a randomizer that only shows ads from one of the two engines at a time. Barry Schwartz has a helpful post over at SE Roundtable that explains step-by-step how to use PHPAdsNew to switch AdSense and YPN ads out on your site.