Unit 8 · Detecting Reasoning Errors
Ms Walker's Milk Study — Confounding Variable
Lesson 16 of 34
About this lesson
Ms Walker splits her class by birthday to test whether milk improves times tables. The milk group scores higher — so milk works, right? Wrong. Children born in January are older than children born in December and may already be better at maths. The experiment was flawed before it started.
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Question
Ms Walker would like to investigate whether drinking milk with lunch makes any difference to her students’ results in a times tables test. She decides to do an experiment.
She splits her class into two groups according to their birthdays. For one whole month, students who were born between January and June are given milk with their lunch, and students who were born between July and December are given juice.
After one month she gives the whole class a times tables test. On average, the students who drank milk with lunch score five marks better than the students who drank juice.
Ms Walker said: “So it’s true then – drinking milk with lunch improves children’s ability to remember the times tables!”
Which one of the following sentences shows the mistake Ms Walker has made?
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