In a 1993 study published in the Journal of Personality and Social Psychology (Vol. 64, No. 3), Ambady and a colleague videotaped 13 graduate teaching fellows as they taught their classes. She then took three random 10-second clips from each tape, combined them into one 30-second clip for each teacher and showed the silent clips to students who did not know the teachers. The student judges rated the teachers on 13 variables, such as "accepting," "active," "competent" and
"confident." Ambady combined these individual scores into one global rating for each teacher and then correlated that rating with the teachers' end-of-semester evaluations from actual students.
"We were shocked at how high the correlation was," she says. It was 0.76. In social psychology anything above 0.6 isconsidered very strong.
Curious to see how thin she could make her slices before affecting the student judges' accuracy, Ambady cut the length of the silent clips to 15 seconds, and then to six. Each time, the students accurately predicted the most successful teachers.
"There was no significant difference between the results with 30-second clips and six-second clips," Ambady says.