To assess the effectiveness of a kindergarten-readiness program, 15 children from a random sample were each given a diagnostic assessment before beginning the program and a follow-up assessment after completing the program. For each child, the difference in the score points between the two assessments was calculated and used to create the 95 percent confidence interval (20. 1,23. 9). Assuming all conditions for inference are met, which of the following is a correct interpretation of the interval?