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Distinguish between observational studies and experiments, and understand causation vs. association.
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Only well-designed experiments can establish cause-and-effect relationships.
Observational studies can show association but cannot prove causation because of potential confounding variables.
A confounding variable is a variable that:
This makes it impossible to determine which variable is actually causing the observed effect.
: People who exercise more tend to weigh less. But people who exercise may also eat healthier. Diet is a confounding variable.
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Avoid these 3 frequent errors
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A lurking variable is not among the explanatory or response variables but may influence the relationship between them.
A trend that appears in several groups of data reverses when the groups are combined. This occurs because of a lurking variable.
Classic example: A treatment may appear better overall, but worse in every subgroup, due to unequal group sizes.
| Term | Definition |
|---|---|
| Explanatory variable | The variable we think explains or causes changes |
| Response variable | The variable we measure as an outcome |
| Confounding | Two variables whose effects cannot be separated |
| Lurking variable | A hidden variable that affects the relationship |
AP Tip: If asked "can we conclude causation?", the answer is NO for observational studies. Always mention confounding variables as the reason.