A drug manufacturer is testing a new headache drug. They recruit 100 subjects with headaches and have them rate the severity of their headaches on a scale of 1 to 10. Based on the severity of the headache, the researchers assign 50 subjects to receive the new drug and 50 to receive a placebo. One hour later the subjects again rate the severity of their headaches. They find that 60% of the subjects had substantial reduction in the severity of their headaches in the treatment group compared to only 30% in the control group. What fundamental component is missing in this experimental design?