The science of landing page optimisation

In recent weeks I have been working with Goggles Website Optimizer. What a great tool for conducting A/B tests or multi-variate tests! An of course it is free.

What I like is just how easy it is to set up. If you have basic HTML experience, then you will not find it too difficult.

What I am struggling with though is how to make changes to the pages once a test has started. For example, I want to add a picture to my page in a section that is not being tested but no matter what I do, I cannot seem to get the new page to load.

If anyone has worked this out, I would like to know. Also, what are your experiences with Googles Website Optimizer?