Nimin has volunteered to participate in an experiment evaluating the effectiveness of aspirin. Neither he nor the experimenters know whether the pills he takes during the experiment contain aspirin or are merely placebos. The investigators are apparently making use of

double blind procedure

Explanation:

Double-blind procedure: In psychological experiment or research, the term "double-blind procedure" is described as a procedure whereby participants and experimenters or researchers are being "blind" to certain important aspects of the research, study, or experiment i.e., they're unaware or without knowledge related to the same, and it involves expectations, hypotheses, and most importantly the assignments of different participants to "experimental group".

In the question above, the investigators are making use of "double-blind procedure".

answer; higher rate;