Patients with Alzheimer's disease are randomly divided into two groups. One group is given a new drug, and the other is given a placebo. After six months they are given a memory test to see whether the new drug fights Alzheimer's better than a placebo.

This is a controlled experiment.

Step-by-step explanation:

A controlled experiment is one in which all variables are kept constant except one, which is the independent variable. The samples are divided into the experimental and the control groups. The control group is the normal state of the condition being tested and would be compared to the experimental groups to check for changes.  

In the experiment above, the experimental group is given the new drug while the group that was administered the placebo is the control group. The two groups are compared after six months to check for changes.

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