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In an observational study, students who chose to use a revision app scored a mean of 9 marks more…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
In an observational study, students who chose to use a revision app scored a mean of 9 marks more on a test than students who did not. Later, a school runs an experiment with random assignment, and finds that the app really adds 4 marks. How many marks of the first gap came from the kind of students who chose the app? What percentage of the first gap is that? Give the second answer to one decimal place.
Need a reminder? The key ideaShow
A survey asks, an observational study watches, and an experiment imposes a treatment. The explanatory variable is the possible cause and the response variable is the outcome. In an observational study, gap seen = real effect + the part due to who chose.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The part due to who chose. marks.
Step 2. As a share. .
Answer: 5 marks, about of the gap. More than half of the gap came from the students and not from the app.
Watch out. The observational study was not useless. It suggested an effect, and the experiment measured it fairly.
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