As part of a study, 20 students rated their own attractiveness on a scale from 1-10, and had a picture taken. In the second part of the study, a group of 79 college student volunteers (not part of the original group of 20) rated the attractiveness of each of the 20 students in the pictures on a scale of 1-10. Researchers averaged the attractiveness rating of the 79 "raters" (referred to as the societal rating) to compare to the self-rating to determine whether or not college students are accurate in their self-perceptions. What is the observed statistic used for a hypothesis test