Explain why some studies include both a control group and a placebo group. What additional comparisons are possible if both a control group and a placebo group are included?
a. Studies that include a placebo group and a control group allow researchers to measure the magnitude of the placebo effect by comparing the control to the treatment(s).
b. Studies that include a placebo group and a control group allow researchers to measure the magnitude of the treatment effect(s) by comparing the placebo to the control.
c. Studies that include a placebo group and a control group allow researchers to measure the magnitude of the placebo effect by comparing the placebo to the treatment(s).
d. Studies that include a placebo group and a control group allow researchers to measure the magnitude of the placebo effect by comparing the placebo to the control.