There is no reason something like A/B testing should give you any problem, but the truth is some sites for administering such tests end up complicating it all beyond belief. Strange. And frustrating.
But that is not the case here. SimpleAsAB is a site that plays things safe, and that lets you conduct A/B tests in the way they should be conducted. On the site, you simply set down the two variables that you want your test subjects to choose from, and then the A/B test will become displayed on your site or blog once you have pasted three lines of Javascript.
SimpleAsAB works with a credit system. You buy as many credits as people you think would do the test. If you fall short of the mark, then you simply buy a couple more and then everybody will be able to do the test. There are no long-term commitments of any kind here. You pay as you go. And you stop when and where you want.
SimpleAsAB.com In Their Own Words
Affordable and powerful A/B testing.
Why SimpleAsAB.com It Might Be A Killer
The fact that you can pay for the service as you go will be enough to win over those who are dead against signing contracts and the like.
Some Questions About SimpleAsAB.com
How much is a credit worth? Are there bulk packages available? 
