I'm a bit of a math geek, and love a good analysis. So one of my favorite posts was the mega-article I published on the Marketo blog: Landing Page Testing – The Ultimate Guide To Test Statistics.
In this article, I explain all the math behind statistical confidence for online tests, including standard deviation, binomial distributions, hypothesis testing, Z-values, Chi-Square distributions, two-sided tests, and more.
And for those who are "allergic" to math, I also share some simple "rules of thumb":
Between 25 to 50 conversions are required to be somewhat confident in a given landing page’s reported conversion rate.
Typically, you need 25 to 50 conversions per test version to be somewhat confident in your test results.
To get the number of versions you can confidently test, take the number of conversions you get per day and divide it by 20. Then take your testing period in weeks. Multiply the two results together, and you’ll estimate the number of versions you can confidently test.
For the mathematically inclined, it's a "fun" post to read. Check it out: Landing Page Testing – The Ultimate Guide To Test Statistics.