The people who are the most likely to take sleeping pills rather than practice behavior modification techniques are those who have previously had the most trouble falling asleep. People who do not take sleeping pills spend at least as many total hours asleep each night as do the people who take sleeping pills. Most people who have trouble falling asleep and who use behavior modification techniques fall asleep more slowly than do most people who have no trouble falling asleep. The people who are the most likely to practice behavior modification techniques rather than take sleeping pills are those who prefer not to use drugs if other treatments are available.
Among people who have a history of chronic trouble falling asleep, some rely only on sleeping pills to help them fall asleep, and others practice behavior modification techniques and do not take sleeping pills. Those who rely only on behavior modification fall asleep more quickly than do those who rely only on sleeping pills, so behavior modification is more effective than are sleeping pills in helping people to fall asleep.
Which one of the following, if true, most weakens the argument?