A study made by a psychologist shows that spending too much time in front of a computer causes insomnia. Two groups of adults took part in this study. During the study the first group spent 3 hours or less per day working on a computer; the second group spent 6 hours or more working on a computer. A greater proportion of the second group had trouble falling asleep during the period of the study than members from the first group.
Which of the following, if true of the adults in the study, most challenges the psychologist's conclusion?
Which of the following, if true of the adults in the study, most challenges the psychologist's conclusion?