<P> For example, in experiments where crop yield is affected (e.g. soil fertility), the experiment can be controlled by assigning the treatments to randomly selected plots of land . This mitigates the effect of variations in soil composition on the yield . </P> <P> In blind experiments, at least some information is withheld from participants in the experiments (but not the experimenter). For example, to evaluate the success of a medical treatment, an outside expert might be asked to examine blood samples from each of the patients without knowing which patients received the treatment and which did not . If the expert's conclusions as to which samples represent the best outcome correlates with the patients who received the treatment, this allows the experimenter to have much higher confidence that the treatment is effective . </P> <P> The blinding eliminates effects such as confirmation bias and wishful thinking that might occur if the samples were evaluated by someone who knew which samples were in which group . </P> <P> In double - blind experiments, at least some participants and some experimenters do not possess full information while the experiment is being carried out . Double - blind experiments are most often used in clinical trials of medical treatments, to verify that the supposed effects of the treatment are produced only by the treatment itself . Trials are typically randomized and double - blinded, with two (statistically) identical groups of patients being compared . The treatment group receives the treatment, and the control group receives a placebo . The placebo is the "first" blind, and controls for the patient expectations that come with taking a pill, which can have an effect on patient outcomes . The "second" blind, of the experimenters, controls for the effects on patient expectations due to unintentional differences in the experimenters' behavior . Since the experimenters do not know which patients are in which group, they cannot unconsciously influence the patients . After the experiment is over, they then "unblind" themselves and analyse the results . </P>

When is control not needed in an experiment