Medical researchers often test medicines on patients who have given informed consent for the tests. The researchers divide the patients into two groups. One group is given the medicine. The second group is given a harmless substitute such as sugar pills. Neither the patients nor the researchers know which patients have received the medicine and which have received the substitute.

What is the purpose of having neither the patients nor the researchers know who is receiving which treatment?
to increase accuracy
to increase precision
to decrease bias
to decrease patient safety