Experimental psychological research is conducted in the laboratory under controlled conditions. This method of research attempts to rely solely on an application of the scientific method to understand behavior and mental processes. Examples of such measurements of behavior include reaction time and various psychometric measurements. Experiments are conducted to test a particular hypothesis.
As an example of a psychological experiment, one may want to test people's perception of different tones. Specifically, one could ask the following question: is it easier for people to discriminate one pair of tones from another depending upon their frequency? To answer this, one would want to disprove the hypothesis that all tones are equally discriminable, regardless of their frequency. (See hypothesis testing for an explanation of why one would disprove a hypothesis rather than attempt to prove one.) A task to test this hypothesis would have a participant seated in a room listening to a series of tones. If the participant would make one indication (by pressing a button, for example) if they thought the tones were two different sounds, and another indication if they thought they were the same sound. The proportion of correct responses would be the measurement used to describe whether or not all the tones were equally discriminable. The result of this particular experiment would probably indicate better discrimination of certain tones based on the human threshold of hearing.
Experimental psychological research is not restricted to the laboratory. In a natural setting, a manipulation of the situation can be made to see what is its effect on behavior. For example, one piece of research in environmental psychology measured the littering behavior of picnicers in a park as the result of litter already at the site. On random hours and random days, when the picnic area was vacant, the researchers made the area clean or left trash, and then observed how the site was left after the next picnicers visited.
The key phrase to describe an experiment is similar people in different situations. The similarity of the people is achieved by random assignment (This resembles the phrase 'random selection', but they are very different in meaning and in practice). The different situations are created by manipulation of some situational variable. In the previous paragraph, that variable was the absence or presence of litter when the picnicers arrived.