The classic true experiment is one where the researcher has control over all aspects of the experimental situation. Groups of appropriately selected subjects are assigned at random to experimental conditions. Sometimes this type of control isn't possible. In that case, you can use a nonexperimental research design. What kind of situation might require a nonexperimental approach?

Let's say you wanted to test memory after some traumatic event. How about the details of what occurred during a nuclear reactor mishap in your community? In a True Experiment we would select subjects at random and then randomly assign them to a control group (live in a calm environment - Group 1) or an experimental group (we arrange for you to live next to a nuclear incident - Group 2).