ACT Preparation / Unit 20 ACT 1920 worked example, 60 to 90 seconds
Experiment or Observational Study?
The idea
A study is classified in three steps. First, ask whether the researchers assigned the treatment or condition, or found subjects already in it. Second, if they assigned it, ask whether the assignment was random. Third, classify: no assignment means an observational study, which can show association; random assignment means a randomized experiment, which is designed to test cause.
In practice
Worked example: invented study one surveys 500 adults about coffee and sleep and finds that coffee drinkers sleep less. It is observational, so it shows association only. Invented study two randomly gives 100 adults either coffee or decaffeinated coffee each evening for a week and measures their sleep. It is a randomized experiment, so a difference in sleep can be attributed to the caffeine.
Where people slip
An answer claiming an observational study proves a cause overstates it; an answer claiming a randomized experiment shows only association understates it.
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