How can I test a search engine's effectiveness?

Hi all,

Not sure if this is the correct place to post this question but here goes:

I want to find out which search engines are most effective for users in terms of usability, and quality of results.

I am going to be using eye-tracking tools to monitor users when they are using search engines such as Google, Yahoo, MSN and a meta-search engine (maybe Dogpile.com).

Do you have any suggestions on how I could formulate...