Distinguishing association from causation
A survey finds that students who attend voluntary study groups report higher grades. The author concludes that study groups cause higher grades for every student. What is the most important limitation?
A
Higher grades necessarily caused every student to join a study group.
B
A causal conclusion is guaranteed whenever two variables are measured together.
C
Students choose whether to attend, so motivation or other differences could help explain the association.
D
Survey evidence can never inform any argument.
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