1. Twenty depressed patients were treated using cognitive behavioural therapy. Over the course of the six-week treatment, each patient’s mood was monitored every week using a self-report mood scale (where a score of 20 = extremely positive mood and a score of 0 = extremely negative mood). Each week they also completed a quality of sleep questionnaire which was scored from 10 = excellent sleep to 0 = very poor sleep.

At the end of the study the researchers correlated each patient’s final mood score with his or her final sleep score.

Explain how this study could be modified by checking the sleep questionnaire for test-retest ability.