There are two types of experiments, the true experiment
and the quasi-experiment. True experiments are high on internal validity. A true experiment includes several key
a) one or more control groups
b) one or more experimental groups
c) random assignment is the key ingredient for a study to qualify as a true experiment.
In a true experiment the investigator does two things:
a) randomly assigns participants to groups,
like experimental group and control group; and
b) manipulates at least one independent variable.
A quasi experiment is almost the same as true experiment, except now there is no random assignment of participants to groups; only manipulation of the independent variable. In order to reach "cause and effect" conclusions about the effect of the independent variable on the dependent variable, you must use a true experiment.
The true experiment is considered to offer the greatest protection against threats to internal validity. True experiments use randomized choice, selecting subjects and methods in a way that prevents bias in results.
True experiment seeks to demonstrate a cause-and-effect relationship. Quasi-experiment is a research design having some of the characteristics of a true experiment.