EXPERIMENTAL DESIGN

You cannot measure statistical significance without random assignment to control group and treatment group(s).

Experimental control is associated with four primary factors.
  1. The random assignment of individual subjects to comparison groups;

  2. The extent to which the independent variable can be manipulated by the researcher;

  3. The time when the observations or measurements of the dependent variable occur; and

  4. Which groups are measured and how. 

Treatment Group: The portion of a sample or population that is exposed to a manipulation of the independent variable is known as the treatment group.
For example, youth who enroll and participate in recreation programs are the treatment group, and the group to which no recreation services are provided constitutes the control group.  Is this a true experiment?

Validity Issues

There are two primary criteria for evaluating the validity of an experimental design.

Internal validity. Did the treatment make a difference within yout sample?

External validity, If so, is it likely to do the same throughout the population?.

Confounding Variables

Conditions that may confuse the effect of the independent variable with that of some other variable(s).