Statistical Surveys: Bias in a Sample

You want to conduct a study to see if a new drug could improve the condition of patients with schizophrenia.

Half of the patients are given a placebo without knowledge of it being a placebo, and the other half are given the real drug without knowing whether or not it is a placebo or not.

After 2 weeks the surveyor then asks each patient this exact question, "Are you feeling better than you last came in?" The responses to this question are then used to decide whether or not the drug was actually an effective treatment.