A school principal argues: "Our students' average score in the annual test rose this year, so the new teaching method we adopted must be working." Which of the following, if true, would most weaken this argument?

An easier paper accounts for higher scores without any improvement in learning, and the fact that unchanged schools improved equally removes the new method as the explanation. Preparation time, a small enrolment change and the timing of introduction leave the link between method and scores untouched.