1. Experimental Psychology PSY 433
Chapter 3
Experiments – Kinds of Variables

2. Causal Relationship Does a change in X cause a change in Y?
Three components:
Co‑variation of events
Time‑order relationship
Elimination of alternative causes.

3. Kinds of Variables Independent variable (IV) Dependent variable (DV)
Confounding variable – confound
Control variable
Random variable – not in the textbook
Subject variable

4. Independent Variable
The presumed "cause" of a behavioral effect or change in the DV.
Manipulated (varied) by experimenter.
IV has several levels selected by the experimenter.
Occurs, or can be "set up" before the DV is measured.
"Independent" of what the subject does.

5. Dependent Variable
A measurement of behavior -- also a measure of the size of the effect of the IV (the cause of behavior change).
What is recorded by the experimenter.
The behavior occurs after the IV is varied, and DV measures the change in behavior.
"Depends" on manipulation of the IV.
The DV does not have levels.

6. Two Kinds of Influence on DV
Systematic – affects the DV in the same way each time.
Can introduce bias into results.
When it occurs due to the manipulation of the IV we call it an “effect.”
Random – varies and thus affects the DV differently from observation to observation.
Can introduce “noise” into results.
Typically makes it more difficult to observe systematic influences.

7. Confounding Variable
Any variable, besides the IV, that can influence the DV.
A potential cause for the experimental effect, other than the IV.
An alternative explanation for observed findings in a study.
Any variable whose values change systematically across levels of the IV.

8. Alternative Explanations
Does chewing gum help students do better on exams?
What are the confounds?
This is another way of asking, what are the possible alternative explanations for a result?
Chewing gum requires energy because muscles are moving, so there is arousal.
Chewing gum releases sugar into the blood stream, providing more energy.
Sugar may be a reinforcer for learning.

9. Control Variable
A variable whose values remain the same across levels of the IV (e.g., room temp, light levels, time-of-day, etc).
A goal in experimentation is to control as many variables as possible, to eliminate their potential effect on the DV.
Eliminates both confounds and noise.
Except for the IV, each subject should have as closely similar an experience in the experiment as possible.

10. Random Variable
Variable whose values vary randomly in an unbiased way across levels of the IV.
Random variables are usually created by the process of random assignment to levels in the experiment.

11. Subject Variable A personal characteristic
Sex, height, weight, age, education, ethnicity, socio-economic status, etc.
Sometimes called a “demographic” variable.
Data describing subject characteristics should be collected in every study.
Can be controlled or left to vary unsystematically (through random assignment to levels).