To understand is to perceive patterns. There are three types of experimental designs.
Preexperimental designs lack random assignment to the program group and the control group.
This design illustrates some inherent weaknesses in terms of establishing internal validity. The better designs are called true experimental designs and quasi-experimental designs. True experimental designs are more complex and use randomization and other techniques to control the threats to internal validity. The closer your nonequivalent group approximates a true experimental population the stronger the internal validity.