Chapter 2.8, Problem 4E

A study was conducted to determine if the inclusion of a foreign language requirement in high schools may have a positive effect on students’ performance on standardized English exams. From a sample of 100 high schools, 50 of which had a foreign language requirement and 50 of which did not, it was found that the average score on the English proficiency exam was 25% higher for the students having a foreign language requirement. What confounding variables may be present that would invalidate the conclusion that requiring a foreign language in high school increases English language proficiency?