Double blind studies are needed because it eliminates any bias that can arise in the course of testing. If you are conducting a clinical trial for a drug you obviously want it to succeed, and if you know which patients are receiving what treatment then you will be more likely to see positive results in those patients. If we use your example of a new Alzheimer’s treatment. If I am conducting the research and I know which patients have the real treatment and which have the placebo, then there is a chance that I am going to report things more positively in the case of the patient receiving the treatment. Not this might not be a conscious decision, but a single blind study still allows for this kind of bias that might skew results. A double blind study makes sure that this will not happen, and will allow for true results that are reflective of the efficacy of the new treatment.