Practice Confounding Variables in Statistics

Use these practice problems to test your method after reviewing the concept explanation and worked examples.

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Quick Recap

A confounding variable is a third variable that influences both the independent variable and the dependent variable simultaneously, creating a spurious association between them that can be mistaken for a direct causal relationship. Confounders are a major threat to the internal validity of observational studies.

Ice cream sales and drowning deaths correlate. Confounding variable: hot weather. It causes both! Without recognizing confounders, you'd wrongly blame ice cream for drowning.

Showing a random 20 of 50 problems.

Example 1

medium
In a study, students who use a tutor have higher GPAs. Name two confounders that threaten the conclusion that tutoring raises GPA.

Example 2

medium
Researchers find runners have lower blood pressure than non-runners (no assignment). Name two confounders that could fully explain this.

Example 3

medium
A survey claims 'people who eat breakfast earn more.' Identify a confounder and explain how it could fully explain the link.

Example 4

medium
A claim says 'children who play music score higher on IQ tests.' Suggest a confounder and explain why a randomized study would be more convincing.

Example 5

medium
A garden experiment puts all the fertilized plants in the sunny corner. Show why the corner is a confounder and how to fix the design.

Example 6

hard
A research team finds patients given a new surgery die less often than patients given the old surgery. Discover that the new surgery is offered only at large urban hospitals. Identify the confounder and propose a study that would resolve it.

Example 7

medium
A new website redesign launches simultaneously with a holiday sale. Sales rise. Explain why the holiday is a confounder for evaluating the redesign and one experimental design that avoids this.

Example 8

challenge
Country-level data show higher chocolate consumption correlates with more Nobel laureates. Propose the most plausible confounder and explain why this is a textbook confounding (not causation) example.

Example 9

medium
Researchers note coffee drinkers have more heart disease, but coffee drinkers also smoke more. After accounting for smoking, the coffee link vanishes. What does this reveal about smoking's role?

Example 10

medium
In a fertilizer field study, the treated plots were also watered more. Explain how watering confounds the result and one design fix.

Example 11

easy
A confounding variable must be associated with what two things?

Example 12

medium
In which of these is 'gender' a likely confounder: (a) drug A vs drug B trial with random assignment, (b) salary survey comparing engineers and nurses?

Example 13

easy
Which design feature in an experiment is the primary defense against confounding?

Example 14

hard
Define 'Simpson's paradox' in one sentence and explain its connection to confounding.

Example 15

hard
What does it mean to 'control for' a confounder in an observational study?

Example 16

easy
True or false: a confounder must be measurable for it to bias an observational study.

Example 17

hard
In a vitamin study, supplement-takers report fewer colds (no assignment). After matching on age, sex, and exercise, the effect shrinks but persists. Can we conclude vitamins cause fewer colds? Explain.

Example 18

easy
Confounding variables are described as a major threat to what kind of validity in observational studies?

Example 19

hard
A workplace study finds people who attend yoga classes have lower stress. The HR analyst adjusts for income, age, and exercise. Why is the analyst still cautious about claiming yoga causes lower stress?

Example 20

easy
Sales of sunglasses and sunburns both rise in July. What is the most likely confounder?
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