Example of an Interrupted Time-Series Design: Student Absences

What is a primary advantage of the interrupted time-series design over a simple one-group pretest-posttest design?

A researcher measures student anxiety once before introducing a new study-skills workshop and once after. She observes a decrease in anxiety and concludes the workshop was effective. However, if anxiety levels naturally rise and fall from week to week, her conclusion may be unwarranted because a single pre- and post-measurement cannot distinguish a genuine treatment effect from ordinary fluctuations in the outcome variable.