So, this researcher is investigating the impact of singing on the language skills of young children. He takes a group of 100 children (a nice big sample, right?) and teaches them to sing when they are...

So, this researcher is investigating the impact of singing on the language skills of young children. He takes a group of 100 children (a nice big sample, right?) and teaches them to sing when they are 3 years of age and comes back 5 years later and assesses their language skills. He cries “Eureka!” throughout his building once he analyzes the results. What’s wrong here?