"An investigator wants to compare the reading comprehension of first-grade students who are taught reading by treatment A versus first-grade students taught reading by treatment B. He selects all first graders in Washington Elementary School (6 classes, N = 200) and places their names in a hat. The first 60 names taken from the hat are placed in treatment A, and the next 60 names are placed in treatment B. What procedure has the researcher used?"