There are two aspects of the experiment that are required for true experiments.

1. The IV is under the direct control of the experimenter.
   
   
2. The researchers attempt to control extraneous factors that might influence the study results.

One way to control extraneous factors is through Random assignment to the various levels of the IVs. This avoids confounding with other variables. Any factor that participants bring to the experiment that might influence the outcome are randomly distributed across the experimenter groups.

If the experimenter must select subjects from pre-existing groups, the study is a quasi-experiment. Confounding is more of a risk here (see workshop on nonexperimental approaches).