What is a confounding variable?

• A. A variable that measures the outcome
• B. An outside factor that affects both dependent and independent variables, biasing results
• C. A control technique
• D. A type of data visualization

Answer: (D)

A confounding variable is an outside factor that can influence both the independent variable (what you think is causing the effect) and the dependent variable (the outcome you measure). Because it affects both, it can create a false impression that the independent variable is driving the outcome, or it can mask a real relationship.

For example, if you study whether coffee drinking is linked to heart disease, smoking could be a confounder because smokers tend to drink more coffee and smoking itself raises heart disease risk. The observed association between coffee and heart disease could be due to smoking, not coffee, unless you account for smoking.

This concept differs from a variable that simply measures the outcome, a technique used to control bias, or a type of data visualization, all of which don’t describe a factor that biases the relationship between exposure and outcome.