Just to add a little to what BillD has to say on the subject of double blind studies. This methodology is used primarily to test the efficacy of therapies, drugs, treatments etc. ie. does the drug, therapy or treatment have an effect that is SIGNIFICANTLY different from that of a placebo, and by SIGNIFICANTLY I mean by using accepted statistical analysis to measure significance.

Another way to test efficacy is by evaluating the effect something may have by measurment. For example take a blood anti-coagulant, it's efficacy is easily measured by testing it's effect on the amount of time it takes to clot a subject's plasma in a laboratory procedure.

That would be a pretty straightforward way to say that something does what it claims to do. On the other hand if one wants to determine whether this effect has an effect on mortality or morbidity a double blind study would be used to answer that question.