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A teacher tries a new reading programme…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A teacher tries a new reading programme. She puts the 15 students who asked to join into the programme group, and 15 other students into a comparison group. After a term, the programme group's reading scores rose by a mean of 12 points and the comparison group's by 5 points. Find the difference between the groups. Give two reasons why it may overstate the effect of the programme. Then describe a better design.
Need a reminder? The key ideaShow
A fair experiment has a control group, random assignment, a placebo where it helps, blinding and enough subjects. The effect of the treatment is the difference between the two groups, in percentage points or in the units of the response.
The worked solution
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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 difference. points.
Step 2. Reason one: who chose. The students who asked to join are probably keener readers. They may improve faster with or without the programme.
Step 3. Reason two: no blinding. The students knew they were in a special group, and the teacher knew too. Extra attention, and the teacher's hopes when marking, may raise the scores.
Step 4. A better design. List all 30 students and use random assignment to make two groups of 15. If possible, have the reading tests marked by someone who does not know the groups. Give the comparison group a different activity of the same length, so both groups get attention.
Answer: a difference of 7 points, which may include the effects of keener students and of knowing the groups.
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