What is the importance of randomization when assigning participants to the experimental and control groups?

Question

What is the importance of randomization when assigning participants to the experimental and control groups?

Answer 1

Randomly selecting an experimental or control group is an attempt to overcome experimental bias.

For example, if you are trying to test the effectiveness of a new drug, it may be tempting for a researcher to put younger, healthier, and more likable candidates in the experimental group.  These people may have a better chance of beating whatever ailment they have without the drug.

The tendency of this group to be healthier could affect the outcome of the drug trial.