A social worker tracked the depression levels of clients being treated with cognitive-behavioral therapy for depression. For each client, depression was assessed at weeks 1 to 20 of therapy. She calculated a mean for all of her clients at week 1, week 2, and so on, all the way through week 20.

a. What are the independent and dependent variables in this study? 

b. Are the variables nominal, ordinal, or scale?

c. Which graph or graphs would be most appropriate to depict the data? Explain why.