Establishment of the first psychological laboratory at the Leipzig University in the year 1879 certified the use of experimental method in psychology. Physiological psychologists already prepared the groundwork for the use of this method in their efforts to analyze and understand psychological processes involved in sensation, attention, perception, feelings, images, and so on.
Further impetus to the use of experimental method was provided by the psychophysical experiments, which recognized that the relationship between stimulus and the corresponding sensory experience could be studied in the laboratory setting. Hermann Ebbinghaus’s discovery of nonsense syllables and his series of experiments on memory opened a new era in the history of experimental psychology. The classic experiments of E.L.Thorndike, I.P.Pavlov, B.F. Skinner, and W.G. Kohler ensured that the experimental method could be meaningfully applied to the study of behaviors.
By the turn of the 21st century, the use of the experimental method was diversified, and was considered as the predominant method of understanding, explaining, and predicting the psychological processes.
Experimental method is a method in which the investigator systematically manipulates one or more variables to study their effects on other variables in a carefully controlled laboratory setting. Manipulation of variables is made according to a schedule, which is called the design of the experiment. A number of such experimental designs are developed to deal with the queries in psychology. The experimental method is used either to collect data, or to explore a relationship, or to test a hypothesis. If the experimenter wants to explore or find out what would happen by introducing a variable, it is called “exploratory experiment”. If he wants to test a hypothesis or supposition made on the basis of findings of experiments conducted by earlier investigators, it is called a “confirmatory experiment”. A hypothesis is formed on the basis of knowledge available from previous experiments. Usually, hypotheses are not formed relating to exploratory experiments because no provisional answer to the problem under investigation is available in the past studies. But in a confirmatory experiment, there is a definite hypothesis.