The researchers claimed that of those who completed the program, 85% improved. What possible confounds can you identify in this study and how can they be corrected? Or can they?

Initial Post Part 1 (minimum of 250 words)
You read in a health magazine about a study in which a new therapy technique for depression was examined. A group of depressed individuals volunteered to participate in the study, which lasted 9 months. There were 50 subjects at the beginning of the study and 29 at the end of the 9 months. The researchers claimed that of those who completed the program, 85% improved. What possible confounds can you identify in this study and how can they be corrected? Or can they?
Initial Post Part 2 (minimum of 250 words)
Review the internal and external threats to validity in the example from Part 1 of this Forum. Identify and define those threats that are related to measurement, and the threats that are related to sampling issues.