One type of research method in which the investigator manipulates one or more independent variables (IV) to determine the effect(s) on some behavior (the dependent variable) while controlling other relevant factors. There are two types of experiments, the true experiment and the quasi experiment. A true experiment occurs when the investigator does two things: 1) randomly assigns participants to groups (e.g., experimental and control); and 2) manipulates at least one IV. A quasi experiment is almost the same, except now there is no random assignment of participants to groups; only manipulation of the IV. In order to reach "cause and effect" conclusions about the effect of the IV on the DV, you must use a true experiment.