Download Experimental Psychology PSY 433

Experimental Psychology
PSY 433
Chapter 3
Experiments – Kinds of Variables
Causal Relationship
 Does a change in X cause a change in Y?
 Three components:
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Co-variation of events
Time-order relationship
Elimination of alternative causes.
Kinds of Variables
 Independent variable (IV)
 Dependent variable (DV)
 Confounding variable – confound
 Control variable
 Random variable – not in the textbook
 Subject variable
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.
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.
Two Kinds of Influence on DV
 Systematic – affects the DV in the same way
each time.
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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.
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Can introduce “noise” into results.
Typically makes it more difficult to observe
systematic influences.
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.
Alternative Explanations
 Does chewing gum help students do better
on exams?
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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.
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.
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Eliminates both confounds and noise.
 Except for the IV, each subject should have
as closely similar an experience in the
experiment as possible.
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.
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).