Designing psychological research: 11.1.11 natural experiment

The idea

A natural experiment is a research method where the independent variable (IV) is not manipulated by the researcher but changes naturally in the environment. The researcher simply measures the effect of this naturally occurring IV on a dependent variable (DV). Because the researcher does not control the IV or randomly allocate participants to conditions, natural experiments often have lower internal validity but higher ecological validity than laboratory experiments. For example, a researcher might study the effect of a sudden school closure (the naturally occurring IV) on students' stress levels (the DV). This method is particularly useful for investigating situations where it would be unethical or practically impossible to manipulate the IV directly, such as studying the psychological impact of a natural disaster.

What earns the marks

• Identify that the independent variable in a natural experiment occurs naturally and is not manipulated by the researcher.
• Recognise that participants cannot be randomly allocated to conditions in a natural experiment.
• Evaluate natural experiments as having high ecological validity because they study real-world events.
• Explain that natural experiments have lower internal validity due to a lack of control over extraneous variables.

Common mistakes and how to put them right

• Confusing a natural experiment with a field experiment.

  Put it right: In a field experiment, the researcher manipulates the IV in a natural setting, whereas in a natural experiment, the IV changes naturally without researcher intervention.

• Assuming natural experiments have high internal validity.

  Put it right: Natural experiments have low internal validity because the researcher cannot control extraneous variables or randomly allocate participants.

• Believing researchers cannot measure a dependent variable in a natural experiment.

  Put it right: The researcher still measures a dependent variable to assess the impact of the naturally occurring independent variable.

Exam tips

• When defining a natural experiment, always emphasise that the independent variable is naturally occurring and not manipulated.
• Use the lack of random allocation as a key evaluation point when discussing the limitations of natural experiments.
• Be prepared to identify the independent and dependent variables in a scenario describing a natural experiment.