What does "Independent Variable" mean?

What is an Independent Variable?

An independent variable is any variable that could not be influenced by the intervention we are testing. In a typical A/B test these could be a user's age, sec, location, internet speed, internet service provider, and so on. Such variables are usually used for segmentation and targeting. In an online controlled experiment the group to which a user is assigned is an independent variable decided on through randomization.

Contrast this to a dependent variable such as a conversion rate, bounce rate, purchase rate and so on which a conversion rate optimization expert can and hopefully is able to influence in a positive manner.

The requirement for variables to be independent is key in defining a statistical null hypothesis. Departures from independence can be established through mis-specification testing and if detected, a significance test that relies on this assumption is no longer adequate. The requirement for the variable being independent is often accompanied by the condition that they are also identically distributed.

Like this glossary entry? For an in-depth and comprehensive reading on A/B testing stats, check out the book "Statistical Methods in Online A/B Testing" by the author of this glossary, Georgi Georgiev.