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A school garden club has 40 young bean plants in pots, all of one kind…
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A school garden club has 40 young bean plants in pots, all of one kind. Ama wants to know whether a new plant food makes the plants grow taller in four weeks. Describe a fair experiment. Name the treatment, the control group, how the plants are assigned, what is measured and how blinding could help.
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 groups. Give the treatment, the new plant food, to 20 plants. The other 20 plants are the control group and get plain water in the same amount.
Step 2. Random assignment. Number the pots 1 to 40 and use random.sample(range(1, 41), 20) to choose the treatment pots. Keep all the pots in the same place, so they get the same light and warmth.
Step 3. What to measure. The response is the height of each plant after four weeks, in cm. Compare the mean heights of the two groups.
Step 4. Blinding. Ask a friend who does not know which pots got the food to measure the heights. Then hopes cannot change the readings.
Step 5. Enough plants. Twenty plants in each group lets the luck of single plants even out.
Answer: a randomised, blinded experiment with a control group, as above.
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