If we have learned to solve problems by one method, we often have difficulty bringing new insight to similar problems. However, electrical stimulation of the brain appears to help subjects come up with fresh insight.
In a recent experiment1 conducted at the University of Sydney in Australia, 40 participants were trained to solve problems in a certain way and then asked to solve an unfamiliar problem that required fresh insight. Half of the participants were randomly assigned to receive noninvasive electrical stimulation of the brain while the other half (control group) received sham stimulation as a placebo. The participants did not know in which group they were. In the control group, 20% of the participants successfully solved the problem while 60% of the participants who received brain stimulation solved the problem.
(a) Is this an experiment or an observational study?
(b) From the description, does it appear that the study is double-blind, single-blind, or not blind?

(a) Experiment

(b) Single Blind Study

Step-by-step explanation:

(a) Experiment

(b) Single Blind Study

The difference between a single-blind study and double-blind study is that, in a single-blind study, the participants in the clinical trial do not know if they are receiving the placebo or the real treatment but the experimenter knows while in a double-blind study, both the participants and the experimenter do not know which group got the placebo and which got the experimental treatment.  

From the description of the experiment, half of the participants were randomly assigned to receive noninvasive electrical stimulation of the brain by the experimenter while the other half (control group) received sham stimulation as a placebo. This implies that the experimenter knows the group that receives the placebo but the participants did not know which group they were. Therefore, the study is a single-blind study.

what is the starting amount? the question is not clear sorry.

step-by-step explanation: