A title, description and display URL packed into about 70 characters—that's the extent of a paid search listing. Differentiation within these tight boundaries is tricky, to say the least. That's why the best place to start when developing paid search ads is inside searchers' heads.
While there aren't too many elements within a paid search listing to test, there are myriad front-end and back-end variables that affect results-from search engine to ranking, landing page to conversion process.
The following are a few metrics search experts find indispensable to determining the effectiveness of paid search programs.