Distinguish observational studies from experiments.
Experiments assign treatments; observational studies just record what already happened.
核心知识
An experiment actively assigns participants to treatment conditions. An observational study measures what already exists without intervening.
深入理解
The single question that separates experiments from observational studies: did the researcher control who got which treatment?
In an experiment, participants are placed into groups by the researcher — typically through random assignment. One group receives a treatment, another gets a control or different treatment, and outcomes are compared.
In an observational study, participants fall into groups based on their own behavior, characteristics, or circumstances. The researcher just records data without changing anything.
A study where a doctor assigns half the patients to a new drug and half to a placebo is an experiment. A study where a researcher surveys people who already take the drug and people who don't is observational.
On the SAT, look for keywords:
- Experiment signals: "randomly assigned," "were given," "received treatment," "divided into groups"
- Observational signals: "were surveyed," "researchers recorded," "data was collected from," "were asked about their habits"
分步讲解
- Read the study description and ask: did the researcher assign people to groups, or did people end up in groups on their own?
- If the researcher assigned groups → experiment.
- If people were already in groups or chose their own behavior → observational study.
常见误解
- Thinking any study with two comparison groups is an experiment. Observational studies also compare groups — the difference is how people ended up in those groups.
- Confusing surveys with observational studies exclusively. A survey is a data-collection method; it can be part of either an experiment or an observational study.
- Believing large sample size turns an observational study into an experiment. Design, not size, determines the study type.
示例解析
A university researcher selected 300 students and recorded how many hours each student slept per night over a semester. At the end of the semester, the researcher compared the grade point averages of students who averaged more than 7 hours of sleep per night with those who averaged fewer than 7 hours. Which of the following best describes this study?
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