There are two types of experiments, the true experiment and the quasi-experiment. True experiments are high on internal validity. True experimental research is the most accurate form of experimental research design as it relies on statistical analysis to prove or disprove a hypothesis. The term true experiment is sometimes used to refer to any randomized experiment. a 'true experiment' is a method of social research in which there are two kinds of variables. The independent variable is manipulated by the experimenter, and the dependent variable is measured.
In a true experiment, participants are randomly assigned to either the treatment or the control group, whereas they are not assigned randomly in a quasi-experiment. The term true experiment is also used to describe all studies with at least one independent variable that is experimentally manipulated and with at least one dependent or outcome variable. The word true in true experiment has been interpreted to mean there are a limited number of correct experimental methods.
Regardless of the exact definition, the distinguishing feature of true experimental designs is that the units of study are randomly assigned to different treatment conditions. The term "experiment" usually implies a controlled experiment, but sometimes controlled experiments are prohibitively difficult or impossible. In this case researchers resort to natural experiments or quasi-experiments. - Dunning, Thad (2012). Natural experiments in the social sciences : a design-based approach. Cambridge: Cambridge University Press.
A true experiment includes several key features:
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. There is generally little loss of status or prestige in doing a quasi-experiment instead of a true experiment, although you may run into someone who is biased against quasi-experiments. In quasi-experiment, matching instead of randomized testing is used.
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.