In your case, Let us say that there is a new pill developed to treat Alzheimer’s. In the clinical trials, generally with an existing medicine or placebo, the new pill is tested and is it will be a randomized control test. If the doctors do not know which participant gets what, the results concluded by them are based on the findings from the test but without any information about the drug which was administered to the participants. Now, these results are analyzed by the investigators and hence not biased. Now with the single-blind case, The doctors would be expecting results based on the drugs that are administered the groups. This creates a bias in the study and unexpected results or outcomes are generally overlooked. Also, adding onto Ibraheem’s post, double-blind study is feasible if the study involved in many investigators or labs. When its a study only in a single lab, more often than not, the investigator gets information and hence the test becomes single-blinded.