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.
The statement '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' most weakens the argument.