What’s a Confounder?

If you are a student of statistics, you may already know what a ‘confounder’ or “confounding variable” is.

The textbook definition (as found in StatisticsHowTo.com): “In an experiment, the independent variable typically has an effect on your dependent variable. For example, if you are researching whether lack of exercise leads to weight gain, lack of exercise is your independent variable and weight gain is your dependent variable. Confounding variables are any other variable that also has an effect on your dependent variable.”

Got it? No? Okay, here’s an example (edited) that should help. It’s from  Bob Spunt writing at Psychology in Action. 

It is known that throughout the year, murder rates and ice cream sales are highly positively correlated. That is, as murder rates rise, so does the sale of ice cream. There are three possible explanations for this correlation:

Possibility #1: Murders cause people to purchase ice cream.