Understand why random assignment supports causal claims.
Random assignment makes cause-and-effect claims possible.
核心知识
Random assignment balances out all other variables between groups, so any difference in outcomes can be attributed to the treatment itself.
深入理解
Suppose students who sleep more also tend to exercise more, stress less, and eat better. If you just compare sleepers to non-sleepers, you can't tell whether higher grades come from sleep or from those other habits. These hidden factors are called confounding variables.
Random assignment solves this. When you randomly split 200 people into two groups, the exercisers, the stressed-out, the healthy eaters — all of them get scattered roughly evenly across both groups. The only systematic difference left is the treatment you applied.
That's why experiments with random assignment can claim causation: "The treatment caused the difference." Without random assignment, there's always the possibility that some lurking variable — not the treatment — explains the result.
On the SAT, if a question describes random assignment and asks what conclusion is valid, a cause-and-effect statement is usually the right answer. If random assignment is missing, any answer claiming causation is going too far.
分步讲解
- Check whether the study used random assignment to place participants into groups.
- If yes → confounding variables are balanced out → a causal conclusion can be drawn.
- If no → confounding variables may differ between groups → only an association can be claimed.
常见误解
- Confusing random sampling (how you pick people from a population) with random assignment (how you split people into treatment groups). They serve different purposes.
- Thinking that controlling for one variable is enough to claim causation in an observational study.
- Believing that a strong correlation in an observational study is evidence of causation.
示例解析
A researcher randomly assigned 150 adults to either a Mediterranean diet group or a standard diet group for 6 months. At the end of the study, the Mediterranean diet group had significantly lower blood pressure on average. Which conclusion is best supported?
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