Experimental Methods in Psychology

The prime method of enquiry in science is the experiment. The key features are control over variables, careful measurement, and establishing cause and effect relationships. An experiment is an investigation in which the independent variable is manipulated (or changed) in order to cause a change in the dependent variable.

There are three types of experiments you need to know:

1. Laboratory / Controlled Experiments

This type of experiment is conducted in a well-controlled environment – not necessarily a laboratory – and therefore accurate measurements are possible. The researcher decides where the experiment will take place, at what time, with which participants, in what circumstances and using a standardised procedure. Participants are randomly allocated to each independent variable group. An example is Milgram’s experiment on obedience.

• Pro: It is easier to replicate (i.e. copy) a laboratory experiment.

• Pro: They allow for precise control of extraneous and independent variables.

• Pro: They allow cause and effect relationships to be established.

o Con: The artificiality of the setting may produce unnatural behaviour that does not reflect real life, i.e. low ecological validity.

o Con: Demand characteristics or experimenter effects may bias the results and become confounding variables.

2. Field Experiments

Field Experiments are done in the everyday (i.e. real life) environment of the participants. The experimenter still manipulates the IV, but in a real-life setting (so cannot really control extraneous variables), e.g. Holfing’s Hospital Study on Obedience.

• Pro: Behaviour in a field experiment is more likely to reflect life real because of its natural setting, i.e. higher ecological validity than a lab experiment.

• Pro: There is less likelihood of demand characteristics affecting the results, as participants may not know they are being studied.

o Con: There is less control over extraneous variables that might bias the results. This in turn makes the experiment harder to replicate.

o Con: They are more difficult to replicate compared to lab experiments.

3. Natural Experiments

Natural Experiments are conducted in the everyday (i.e. real life) environment of the participants but here the experimenter has no control over the IV as it occurs naturally in real life, e.g. Hodges and Tizard's attachment research (1989) which compared the long term development of children who have been adopted, fostered or returned to their mothers with a control group of children who had spent all their lives in their biological families.

• Pro: Behaviour in a natural experiment is more likely to reflect life real because of it natural setting, i.e. very high ecological validity.

• Pro: There is less likelihood of demand characteristics affecting the results, as participants may not know they are being studied.

• Pro: Can be used in situations in which it would be ethically unacceptable to manipulate the independent variable.

o Con: They may be more expensive and time consuming than lab experiments.

Experiment Definitions

Ecological validity: The degree to which an investigation represents real-life experiences.

Experimenter effects: These are the ways that the experimenter can accidentally influence the participant through their appearance or behaviour.

Demand characteristics: the clues in an experiment that lead the participants to think they know what the researcher is looking for (e.g. experimenter’s body language).

Independent variable (IV): Variable the experimenter manipulates (i.e. changes) – assumed to have a direct effect on the dependent variable.

Dependent variable (DV): Variable the experimenter measures.

Extraneous variables (EV) are all variables, which are not the independent variable, but could affect the results (DV) of the experiment. EVs should be controlled were possible.

Confounding variables: Variable(s) that have effected the results (DV), apart from the IV. A confounding variable could be an extraneous variable that has not been controlled.

How to cite this article:

McLeod, S. A. (2012). . Retrieved from