PSYC 3450 Experimental Psychology

Introduction to Experimental
Chapter 5. Introduction to Experimental Research
Chapter Objectives
• Define a manipulated independent variable and
identify examples that are situational, task, and
instructional variables
• Distinguish between experimental and control
• Recognize the presence of confounding variables in
an experiment and understand why confounding
creates serious problems for interpreting the results
of an experiment
Chapter Objectives
• Identify independent and dependent variables, given
a brief description of any experiment
• Distinguish between manipulated independent
variables and those that are subject variables
• Describe the interpretation problems that
accompany the use of subject variables
Chapter Objectives
• Recognize the factors that can reduce the statistical
conclusion validity of an experiment
• Distinguish between the internal and external
validity of a study
• Describe the various ways in which an experiment’s
external validity can be reduced
The meaning of Experiment
• Experiment: a systematic research study in which
the investigator directly manipulates some factor,
holds all other factors constant, and observes the
results of the variation.(contrast with correlation)
• “investigating the effect of X on Y”
Essential Features of Experimental Research
• Establishing independent variables (IVs)
• Manipulated IVs : must have minimum of 2 levels
• Situational: environmental features
• Task: different problems to solve
• Instructional: performing tasks in different ways
• Experimental groups
• given treatment
• Research Example  given a golf ball and told it was a “lucky” ball
• Control groups
• treatment withheld
• Research Example  given a golf ball and not told it was a “lucky” ball
Essential Features of Experimental Research
• Controlling extraneous variables: variables which are not of
interest to the researcher but which might influence the
behavior being studied if not controlled for properly.
• Confounds
• Any uncontrolled extraneous variable
• Covaries with the IV; results could be due to IV or to confound
• Distributed practice example
Essential Features of Experimental Research
• Measuring dependent variables (DVs)
• DVs are any behaviors measured in an experiment
• Review scales of measurement (Ch. 4)!
• Problems:
• Ceiling effects
• task is too easy, all scores very high, disguising any differences
• Floor effects
• Task too difficult, all scores very low, disguising any differences
• Solution:
• Task of moderate difficulty, determined through pilot testing
Manipulated versus Subject Variables
• Subject Variables
• Already-existing attributes of subjects in a study
• Examples  gender, age, personality characteristic
• Anxiety example
• As a manipulated variable  induce different degrees of anxiety
in participants
• As a subject variable  choose participants who have different
degrees of their typical anxiety
Manipulated versus Subject Variables
• Cannot draw certain conclusions when using
subject variables
• With a manipulated IV
• Assuming no confounds  IV causes DV
• With a subject IV
• Groups may differ in several ways  IV cannot be said to
cause DV
• All that can be said  the groups differ from each other
Manipulated versus Subject Variables
• Using both manipulated and subject IVs
• Bandura’s Bobo study (Box 5.2)
• Manipulated  type of exposure to violence
• Subject  gender
Type of aggression, gender
Emotional arousal,
Proportion of aggressor
to Bobo size
Aggression score
The Validity of Experimental Research
Statistical conclusion validity
• Proper statistical analyses and conclusions
Construct validity
• Well-chosen and well-defined IVs and DVs
External validity: the degree to which research findings generalize beyond the
specific context being studied (subject pools, the college sophomore problem,
the male problem)
Internal validity- methodologically sound and confound-free (confidence that
IV is directly responsible for results)
Threats to Internal Validity
• Studies extending over time (may have pretests)
• Example: Test-anxiety reduction
History – courses become Pass/Fail
Regression to the mean
Testing and instrumentation- practice effects, change in
measurement instruments/coders
• Importance of using a control group
Threats to Internal Validity
• Participant problems
• Subject selection: method/criterion for participation
produces a confound
• The Brady study  ulcers in executive monkeys
• Attrition
• Loss of data
• Subject selection problem
• Experimental research involves independent and dependent
variable, in an effort to test the effects of the IV on the DV.
• We attempt to control for confounding variables to increase the
internal validity of our study.
• We must consider other possible threats to internal validity as
they pertain to our study.
• Once we identify IVs, DVs, and threats to validity, we design a
study to control those threats.

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