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Experiments
The experimental method allows us to establish cause and effect - the only thing that is different between our groups is the independent variable, so any difference in the dependent variable is due to the independent variable.
Using the experimental method helps us to avoid the effects of confounding variables - a variable other than the one you are interested in that affects the results of your experiment.
Example: If you test all of your experimental group participants in the morning and all of your control group participants in the afternoon, time of day is a confounding variable.
Basic method:
- Use random assignment to assign participants to conditions. The only difference between your groups should be the independent variable(s).
- Manipulate the independent variable(s).
- Measure the dependent variable(s).
- If there is a difference between your groups on the dependent variable(s), it must be due to the manipulation of the independent variable(s)
The sample (group of people included in the experiment) is ideally drawn at random from the population, to make sure it represents the population.
Single-blind and Double-Blind Experiments
If an experiment involves a treatment of some kind, it is common to include a placebo so that participants do not know if they are receiving the real treatment or not.
If the treatment group and control group both show improvement, then it is likely due to a placebo effect - a change that is due to the belief that you are receiving treatment, rather than the treatment itself.
Single-blind experiment - participants do not know whether they are getting the real treatment or a placebo.
Double-blind experiment - participants and the people interacting with the participants do not know whether the participants are getting the real treatment or a placebo.
In many cases, a true experiment is not possible:
- Logistical/practical reasons: some variables cannot be randomly assigned or manipulated, like gender or presence of a disease
- Ethical reasons: some variables cannot be manipulated ethically, like childhood trauma
Watch Out!
If a question describes what sounds like an experiment, but what looks like the independent variable is something that couldn't be manipulated for practical or ethical reasons (gender, childhood trauma, household income, being in a particular class at school, etc.), it is not a true experiment and cause and effect cannot be established.
Mark Yourself Question
- Grab a piece of paper and try this problem yourself.
- When you're done, check the "I have answered this question" box below.
- View the solution and report whether you got it right or wrong.
Practice: Experiments and Studies
You are a researcher in the Western University Psychology department and are interested in studying the effects of listening to classical music on arithmetic skills in undergraduate students. Briefly describe an experiment you could design to study this.
Practice: The Experimental Method
An administrator believes that the placement of posters with reminders to wash hands on the walls in the washrooms of hospitals will lead to increased hand washing in residents. To test his theory, he randomly assigns floors of the hospital to have posters or not have posters and measures how many residents wash their hands on each floor over a week. What is the independent variable in this study?