Let's say I teach a math course with 30 students enrolled. I teach this course for three years, with different students each year. In between years three and four I implement a new academic program. I keep this program for years four, five, and six. At the end of each year, the students take a standardized assessment. I want to compare the collective scores from years 1-3 to the scores from years 4-6 to measure the program's effect, if any.