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Mrs Jones conducts an experiment to investigate whether eating a bar of chocolate during break time improves her students’ problem-solving skills.

She divides her class into two groups; students with the last name starting with A to L in the first group and M to Z in the second group. The first group is given chocolate bars during break time, whereas the second group is given muesli bars.

Mrs Jones gives her students a problem-solving skills test after three weeks and finds on average, the students who ate muesli bars scored 6 marks better than the students who ate chocolate.

Mrs Jones: “So we can conclude then, eating muesli bars during breaks improves children’s problem-solving skills!”

Which of the following statements shows the mistake Mrs Jones has made?