In a clinical trial of drug r for the treatment of generalized anxiety disorder (gad), researchers hypothesize that 200 mg of drug r will reduce gad symptoms by 25 percent, as evidenced by the hamilton anxiety scale. the independent variable is

Drug R will is the independent varaiable.

Explanation:

Drug R in your question is independent variable because it is in your control and you can change the amount of drug i.e. from 200mg to 400mg. As the name independent variable indicates the amount of drug is not dependent on anything.

The GAD  is dependent variable because it is responding to amount of Drug R. The GAD is dependent on amount of Drug R used i.e. if u increases the Drug R amount from 200mg to 400mg, the GAD symptoms will be reduced from 25 % to 50%.

Hamilton Anxiety Scale is constant variable in the experiment because it did not change.

Hoping it might be helpful !!