Extraneous and Confounding Variables

When we experiment, we are trying to determine a cause-and-effect relationship between the explanatory variable (factor) and the response variable (what we are measuing). Sometimes, this is easier said than done because there are extraneous variables that we don't consider when conducting the experiment. Extraneous variables are other factors, like gender, age, weight, etc. that are not directly addressed when the experiment is conducted, but still exist anyway.

All experiments have extraneous variables, but they can become problematic whenever they affect the response variable. These are called confounding variables because they affect what we are measuring. Why don't we want them? Well, if confounding variables affect the response, AND the explanatory affects the response, it is difficult for us to determine exactly WHAT causes changes in the response variable: the confounding variable or the explanatory variable.