Analyze each of the following reports, explaining the way in which the method of agreement and the method of difference have been jointly applied, and identifying the special force, if any, of their combination.

Sixteen-year-old David Merrill, of Suffolk, Virginia, hypothesized that the loud sounds of hard-rock music have a bad effect on its devoted fans. He tested the theory on mice. Seventy-two mice were divided into three groups of 24, the first to be exposed to hard-rock music, the second to music by Mozart, and the third to no music at all.
After allowing the mice to become accustomed to their environments, but before exposing them to the music, Merrill tested all of them in a maze, which took the mice an average of 10 minutes to complete. Then the groups were exposed to the music for 10 hours a day. With repeated testing the control-group mice reduced their time in the maze by an average of 5 minutes. Those exposed to Mozart reduced their time by 8.5 minutes. The hard-rock mice increased their time in the maze by 20 minutes.
Merrill also reported that when, in an earlier attempt, he had allowed all the mice to live together, the project had to be cut short because, unlike the Mozart-listening mice, the hard-rock-listening mice killed other mice.
—Reported in Insight, 8 September 1997