1. Experimental methods Worksheet:
The worksheet 3_Evaluation_of_research_methods.doc can be used as a template for students to evaluate any theory or research study in Research Methods.
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Image © dragon_fang, shutterstock.com
Experimental methods

2. By the end of this section students will be able to:
Learning objectives
By the end of this section students will be able to:
Describe the scientific approach to the study of behaviour
State the main features of a laboratory experiment
State the main features of a field experiment
State the main features of a natural experiment
Icons key:
For more detailed instructions, see the Getting Started presentation
Flash activity (not editable)
Teacher’s notes (in Notes Page)
Accompanying worksheet
Web links
Extension activity
Sound
Video

3. The experimental method
All experiments are designed around the experimental method.
It is a method used by scientists to test hypotheses.
participants randomly allocated to conditions
a change in the dependent variable, which is measured by the experimenter
independent variable manipulated by experimenter
extraneous variables
are controlled
Psychologists use three types of experiments to investigate behaviour: laboratory, field and natural experiments.

4. Laboratory experiment
Reference:
Loftus, E.F. and Palmer, J.C. (1974) Reconstruction of automobile destruction: an example of the interaction between language and memory. Journal of Verbal Learning and Verbal Behavior, 13,
More information about this study can be found in the ‘Eyewitness testimony’ presentation in the Cognitive module of AS Psychology.

5. Field experiment Reference:
Bickman, L. (1974a) The social power of a uniform. Journal of Applied Social Psychology, 4,
More information about this study can be found in the ‘Explanations of obedience’ presentation in the Social module of AS Psychology.

6. Natural experiment Reference:
Hodges, J and Tizard, B. (1989) Social and family relationships of ex-institutional adolescents. Journal of Child Psychology and Psychiatry, 30,
More information about this study can be found in the ‘Privation’ presentation in the Developmental module of AS Psychology.

7. Class discussion
A psychologist wants to investigate the effect of chocolate on the performance of students. How could the psychologist test this?
What variables need to be controlled? How could they be controlled?
What experiments could be performed? Suggest laboratory, field and natural experiments he could perform.
Teacher’s notes:
Here are possible responses to the questions:
The amount of chocolate eaten is the independent variable and the performance of the students is the dependent variable. Extraneous variables that need to be controlled are things that may have an effect on the students performance, such as amount of sleep or time of day, otherwise they may confound the results.
Laboratory experiment: the researcher could invite students to participate in an experiment in his laboratory, where he controls precisely the amount of chocolate they eat and when it is eaten before they complete a standardized test. The advantage is that by being able to control all of the variables he can show any cause and effect between the independent variable and dependent variable. The disadvantage is that in an artificial environment the results may not be applicable to a natural environment, students may pick up on demand characteristics and alter their behaviour.
Field experiment: field experiments are carried out in the natural social settings of the participants. The amount of chocolate students ate and when it was eaten could be controlled whilst students continue their usual classes. The test could be based on the topic of the lesson. The advantage of this is that although the variables are still being controlled, the students are in their natural social setting and so the effect of demand characteristics will be reduced. The disadvantage is that it would be harder to control the extraneous variables so the likelihood of confounding effects is higher.
Natural experiment: the researcher could observe students at two schools, one that allows chocolate on site and one with a healthy eating policy that does not allow chocolate on site. The exam results of the students at each school could be used to compare performance. The advantages of this experiment is that student behaviour will not be altered because they are not being manipulated by the researcher, however, no variables are being controlled so the likelihood of confounding effects from extraneous variables is high and the experiment will be unrepeatable.
What are the advantages and disadvantages of each type of experiment?