Twenty depressed patients were randomly assigned to two groups of ten. Each group spent 2 hours each day working and visiting at the clinic. One group worked in bright artificial light; the other did not. At the end of the study, the mean depression score for those who received light was 6; for those who did not, the mean score was 11. (High scores mean more depression.)
The standard error of the difference was 1.20. Establish a 99 percent confidence interval about the mean difference and write an interpretation.