Experimental method. Types of experiment, laboratory and field…

The idea

An experiment manipulates one variable and measures another while holding the rest steady, so a difference in the dependent variable can be pinned on the independent variable. In a laboratory the researcher controls the setting as well as the manipulation: Loftus and Palmer got mean speed estimates of 40.8 mph for "smashed" against 31.8 mph for "contacted", a difference only that control makes interpretable, though artificial tasks invite demand characteristics. A field experiment moves the same deliberate manipulation into the participants’ own setting, gaining realism and losing control. In a natural experiment the variable changes without the researcher, as with Rutter’s Romanian adoptees; in a quasi-experiment it is a fixed characteristic such as age or sex. Neither allows random allocation, so participant variables may confound the result.

What earns the marks

• The defining features: an independent variable manipulated, a dependent variable measured, other variables controlled, so cause can be inferred.
• Laboratory experiment: manipulation in a controlled, often artificial setting, giving high internal validity, standardisation and easy replication.
• Field experiment: the researcher still manipulates the variable, but in the participants’ natural environment, raising realism and reducing control.
• Natural experiment: the independent variable varies for reasons outside the researcher’s control and would have varied anyway, as in Rutter’s Romanian orphan study.
• Quasi-experiment: the independent variable is a pre-existing characteristic of the participants, such as age, sex or a diagnosis, and cannot be manipulated at all.
• Neither natural nor quasi-experiments permit random allocation, so participant variables are uncontrolled and causal conclusions are weaker.
• Evaluation: laboratory control against low mundane realism and demand characteristics; field realism against uncontrolled extraneous variables and consent problems.

Common mistakes and how to put them right

• Calling any study done outdoors a natural experiment.

  Put it right: The test is who set the independent variable, and in a field experiment the researcher still sets it.

• Using "quasi-experiment" for anything not in a laboratory, when its defining feature is that the variable is a characteristic the participant already had.

• Claiming a laboratory experiment removes extraneous variables, when control reduces them and demand characteristics remain.

• Writing an unoperationalised independent variable, for instance "noise", instead of the two conditions a participant would actually experience.

Exam tips

• When asked to identify the type of experiment, justify it by saying who or what set the independent variable, quoting the stem.
• Operationalise both variables as they would be recorded: the two conditions for the independent variable, the units or score for the dependent variable.
• For "explain one limitation of this natural experiment", go straight to the absence of random allocation and name a plausible participant variable in the scenario.
• Do not evaluate laboratory work as automatically weak; say which conclusion in this scenario is threatened, and why.