EXAMPLE 9 An Uncontrolled Experiment

A college regularly offers a review course to prepare candidates for the Graduate Management Admission Test (GMAT) required by most graduate business schools. However, one year the college decided to offer only an online version of the course. The average GMAT score of students in the online course was 10% higher than the longtime average of those who took the classroom review course. Can we conclude that the online course is more effective?

This experiment has a very simple design. A group of subjects (the students) were exposed to a treatment (the online course), and the outcome (GMAT scores) was observed. The design can be represented as

or, in general form, as