A school found that pupils who joined its new morning reading hour improved their spelling marks over the term more than pupils who did not join. The head teacher concluded that the reading hour improves spelling. Which of the following, if true, does most to undermine that conclusion?

The pupils who joined received something else the others did not — weekly spelling practice at home — and that alone could produce the whole gap in marks, leaving the reading hour with nothing to its credit. Uniform testing supports the finding rather than undermining it, imperfect attendance would blunt any effect rather than create a false one, and the early start describes when the hour was held without explaining the marks.