An elementary school teacher wants to show that parents' involvement helps their children learn. She randomly chooses one half of the boys and one half of the girls in her class and sends a note home with them. The note asks the parents to spend more time each day working with the child on his or her reading homework. The other half of the children do not receive a note. At the end of the school year, the teacher finds that the children whose parents she sent notes to have significantly better final reading grades.

Why can the researcher NOT conclude that parental involvement increased the student's scores?