An observational study records data without imposing treatments, while an experiment deliberately manipulates a variable. Only experiments with random assignment can establish causation; observational studies can only show association.
Observational: you watch people who already smoke and compare their lung cancer rates to non-smokers. Experimental: you randomly assign people to smoke or not (unethical, but illustrates the point). The observational study might find that smokers differ from non-smokers in many ways (diet, exercise, stress)—so you can't be sure smoking caused the cancer. The experiment controls for everything else.
Showing a random 20 of 50 problems.
Example 1
challenge
A newspaper headline reads 'People who eat breakfast weigh less.' Design an experiment that could test causation, naming the key feature that observation lacks.
Example 2
easy
True or false: only experiments can establish causation.
Example 3
hard
Why is it impossible to conduct a fully randomized experiment to test the effect of poverty on health outcomes? What study design limitations result from this constraint?
Example 4
hard
What is Simpson's paradox?
Example 5
easy
True or false: experiments are always the better choice regardless of ethics or practicality.
Example 6
medium
Why is it unethical to randomly assign children to a smoking group to study health effects?
Example 7
medium
Classify: a prospective study that follows a cohort over time, recording but not assigning their exposures.
Example 8
easy
A study records the diets of people who already chose to eat them and tracks health, imposing no treatment. Observational or experimental?
Example 9
challenge
Smoking-cancer causation was established largely from observational data. Explain how scientists strengthened a causal case without a randomized human experiment.
Example 10
medium
True or false: a placebo group is a feature of observational studies.
Example 11
hard
A 1980s study found people who took vitamin E supplements had lower heart disease rates. Later randomized trials found NO effect. Explain the discrepancy.
Example 12
easy
A survey asks 200 people how many hours of sleep they got and their stress level. Type?
Example 13
medium
What is a 'natural experiment'?
Example 14
hard
A study finds a strong correlation between ice cream sales and drownings. Why can't we conclude ice cream causes drowning?
Example 15
easy
True or false: assigning subjects to groups by chance makes a study experimental.
Example 16
medium
A health agency cannot ethically assign people to smoke. What study type must it use, and what is the cost?
Example 17
easy
Researchers randomly assign patients to a new drug or placebo and compare. Observational or experimental?
Example 18
easy
A confounding variable in an observational study is best described as what?
Example 19
easy
Observational studies can only establish ____.
Example 20
hard
Why is a randomized controlled trial (RCT) considered the 'gold standard' for testing medical treatments?