Why is a control necessary in an experiment?

Because without a control then you could not work through the problem.

To make sure that you are testing just one variable.

If you are testing 2 things and something is changed, which of those two things caused the change?

Example: Nurse Johnson gives the experimental drug X to 10 cancer patients and 5 of them are cured.

Nurse Smith gives drug Y to 10 other cancer patients and 10 are cured.

Nurse Smith is very attractive and Nurse Johnson is ugly.

Did the patients do better in the second group because of drug Y or because of nurse Smith being attractive? With 2 variables we can't figure out the real results.

Let's now eliminate one variable and have nurse Johnson give all the shots except one group gets drug X and another group gets drug Y.

Now the X group has 5 out of 10 cured and the Y group has 10 out of 10 cured. Now we now it was the drug effect and not the nurses face that helped the Y group.

Now I can talk about the control group which tries to control all of the variables we can think of except the single one we are testing.

In this cancer lab we will run experiments with 2 groups being treated identical except for the one drug by having equal nurses giving injections to both groups at the same time of day and with the diets the same and the cancer patients have identical health situations. Every thing else will be identical with the 2 groups like smoking history and family history. This gets rid of all variables except the one being tested.

The only difference will be that our experimental group will get the drug in the injection and the control group will get water in the injection. Maybe they should be called the controlled variable group.