Experiment Basics: Variables
Psych 231: Research
Methods in Psychology
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Class Experiment
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Print out the Class experiment (listed on syllabus page)
exercise and bring it to labs this week
Turn in your data sheets (pass to front)
• I will analyze the data and the results will be discussed in
labs this week
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Turn in your consent forms
Quiz 5 (chapter 4) is due Friday
Class Experiment
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Results
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Mean: 77.9%
Median: 79%
Range: 57-93
If you want to go over your exam set up a
time to see me
Exam 1
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Common errors:
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Four Cannons of scientific method (& pg 710 of the textbook)
Exam 1
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Common errors:
A researcher examined the relationship between music and mood. He
presented two groups of participants the same video clips but the two groups
received different musical soundtracks. Following the presentation of the
videos, participants completed a questionnaire designed to measure their
current mood.
(1) Identify the Research Design used in the study
(2) Identify a major advantage of using this research design for this
study
(3) Identify a major disadvantage/limitation of using this research
design for this study.
Experiment: manipulated the music (IV) and measured
the effects on mood (DV).
Major advantage: ability to make causal claims
Major disadvantage: (several possible answers, generalizability)
Exam 1
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You’ve got your theory.
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What behavior you want to examine
Identified what things (variables) you think
affects that behavior
So you want to do an experiment?
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You’ve got your theory.
Next you need to derive predictions from
the theory.
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These should be stated as hypotheses.
In terms of conceptual variables or constructs
• Conceptual variables are abstract theoretical entities
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Consider our class experiment
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Theory & Hypotheses:
• What you try to recognize &
• how connected to your social network you feel.
Social vs. Non-social websites
Cell phone presence
So you want to do an experiment?
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You’ve got your theory.
Next you need to derive predictions from the
theory.
Now you need to design the experiment.
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You need to operationalize your variables in terms of how
they will be:
• Manipulated
• Measured
• Controlled
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Be aware of the underlying assumptions connecting your
constructs to your operational variables
• Be prepared to justify all of your choices
So you want to do an experiment?
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Characteristics of the psychological situations
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Constants: have the same value for all individuals
in the situation
Variables: have potentially different values for
each individual in the situation
Variables in our experiment:
• Connectedness to social network
• Type of website
• Recognition memory performance
• Time for unscrambling
• Kind of cell phone present
•…
Constants vs. Variables
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Conceptual vs. Operational
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Conceptual variables (constructs) are abstract
theoretical entities
Operational variables are defined in terms within
the experiment. They are concrete so that they
can be measured or manipulated
Conceptual
Operational
Social connectedness
Cell phone presence or
absence
Social concepts/words
Websites social or nonsocial
Recognition Memory
Word scramble test
Variables
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Independent variables (explanatory)
Dependent variables (response)
Extraneous variables
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Control variables
Random variables
Confound variables
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Correlational designs
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Many kinds of Variables
have similar functions
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Independent variables (explanatory)
Dependent variables (response)
Extraneous variables
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Control variables
Random variables
Confound variables
Many kinds of Variables
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The variables that are manipulated by the
experimenter (sometimes called factors)
Each IV must have at least two levels
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Remember the point of an experiment is
comparison
Combination of all the levels of all of the IVs
results in the different conditions in an
experiment
Independent Variables
Factor A
1 factor, 2 levels
Condition 1 Condition 2
Factor A
1 factor, 3 levels
Cond 1 Cond 2 Cond 3
Factor B
2 factors, 2 x 3 levels
Cond 1 Cond 2 Cond 3
Factor A
Cond 4 Cond 5 Cond 6
Independent Variables
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Methods of manipulation
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Straightforward
• Stimulus manipulation - different conditions use
different stimuli Social vs. non-social websites
• Instructional manipulation – different groups are given
different instructions
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Staged
• Event manipulation – manipulate characteristics of the
context, setting, etc. Presence or absence of cell phone
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Subject (Participant)– there are (pre-existing mostly)
differences between the subjects in the different conditions
• leads to a quasi-experiment
Manipulating your independent variable
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Choosing the right levels of your independent
variable
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Review the literature
Do a pilot experiment
Consider the costs, your resources, your limitations
Be realistic
• Pick levels found in the “real world”
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Pay attention to the range of the levels
• Pick a large enough range to show the effect
• Aim for the middle of the range
Choosing your independent variable
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These are things that you want to try to
avoid by careful selection of the levels of
your IV (may be issues for your DV as well).
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Demand characteristics
Experimenter bias
Reactivity
Floor and ceiling effects (range
effects)
Identifying potential problems
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Characteristics of the study that may give away the
purpose of the experiment
May influence how the participants behave in the study
 Examples:
• Experiment title: The effects of horror movies on mood
• Obvious manipulation: Having participants see lists of words
and pictures and then later testing to see if pictures or words
are remembered better
• Biased or leading questions: Don’t you think it’s bad to
murder unborn children?
Demand characteristics
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Experimenter bias (expectancy effects)
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The experimenter may influence the results
(intentionally and unintentionally)
• E.g., Clever Hans
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One solution is to keep the experimenter (as well as
the participants) “blind” as to what conditions are
being tested
Experimenter Bias
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Knowing that you are being measured
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Just being in an experimental setting, people don’t
always respond the way that they “normally”
would.
• Cooperative
• Defensive
• Non-cooperative
Reactivity
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A value below which a response cannot be
made
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As a result the effects of your IV (if there are indeed
any) can’t be seen.
Imagine a task that is so difficult, that none of your
participants can do it.
Floor effects
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When the dependent variable reaches a level
that cannot be exceeded
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So while there may be an effect of the IV, that effect
can’t be seen because everybody has “maxed out”
Imagine a task that is so easy, that everybody scores
a 100%
To avoid floor and ceiling effects you want to
pick levels of your IV that result in middle level
performance in your DV
Ceiling effects
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Floor: A value below which a response cannot be made
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Ceiling: When the dependent variable reaches a level
that cannot be exceeded
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As a result the effects of your IV (if there are indeed any) can’t
be seen.
Imagine a task that is so difficult, that none of your participants
can do it.
So while there may be an effect of the IV, that effect can’t be
seen because everybody has “maxed out”
Imagine a task that is so easy, that everybody scores a 100%
To avoid floor and ceiling effects you want to pick levels
of your IV that result in middle level performance in your
DV
Range effects