A study compared the effectiveness of several antidepressants by examining the experiments in which they had passed the FDA requirements. Each of those experiments compared the active drug with a placebo, an inert pill given to some of the subjects. In each experiment some patients treated with the placebo had improved, a phenomenon called the placebo effect. Patients depression levels were evaluated on the Hamilton Depression Rating Scale, where larger numbers indicate greater improvement. The scatterplot at the top of the next column compares mean improvement levels for the antidepressants and placebos for several experiments.

a) Is it appropriate to calculate the correlation? Explain.

b) The correlation is 0.898. Explain what we have learned about the results of these experiments.