Chapter 7
Experiments with One Independent
Variable
Determining Variables of Interest
There are many ways to create and manipulate
variables
• The current nature of society often dictates what
variables are studied and how they are measured
• For example, eyewitness testimony was not studied
for decades but is now a significant area
– How do you measure eyewitness accuracy?
– Confidence is not a good measure
– Speed of response is sometimes a good measure,
but not in real-life, practical situations
Independent and Dependent Variables
Types of Independent Variables
Manipulated variable—True independent variable
that is manipulated by the experimenter and can
lead to random assignment
Measured Variable—A variable that resembles a
true independent variable bit is not; this variable is
used to create discrete groups such that inclusion in
groups is based on a characteristic of the participant
Subject (Participant) Variable—Measured
variable that resembles a true IV but is based on
a pre-existing characteristic of the participant
Independent and Dependent Variables
Types of Studies Using Discrete Groups for Comparison
Experiment—A study in which participants are randomly
assigned to groups
Quasi-Experiment—A study that resembles a true
experiment but involves comparison of groups that are
not formed by random assignment of participants to
groups
Ex Post Facto Study—A study that resembles a true
experiment but uses existing grouped data that did not
involve random assignment of participants to groups
Independent and Dependent Variables
• Research Example of Measured IV
– Question: Do depressed people have worse memories
than non-depressed people?
– Method: Assign people to groups based on preexisting trait (i.e., are they depressed?) and test them
for memory of positive words
– Results: Depressed people remembered fewer
positive words than non-depressed people did
– Conclusion: Memory was poorer for depressed
people, but because they came to the study
depressed, there was no real manipulation of the IV,
so you cannot draw a causal conclusion that
depression hampers memory
Independent and Dependent Variables
• Research Example of a Manipulated IV
– Question: Do suggestions affect memories?
– Method: Randomly assign participants to groups
and tell some participants that dreams indicate
prior experience but do not tell that to other
participants.
– Results: Dream interpretations induce changes in
memories of past events.
– Conclusion: We can influence people’s memories
by giving them erroneous information
Types of Independent and Dependent
Variables
• Qualitative Variable—Variable based on
qualitative differences (e.g., type of
information given to participants) rather than
on quantitative differences.
• Quantitative Variable—Variable based on
quantitative differences like size, duration,
amounts, etc.
Types of IVs and DVs
Example of Study Using Qualitative IV
Percentage of Hallucinatory Reports
Question: Do reports of hallucinatory dreams differ in
different sleep stages?
Method: Awaken people and get dream reports when
they are in qualitatively
different mental states.
Result: Hallucinatory
dreams change in
different sleep states.
100
80
60
40
20
0
Active Wake
Quiet Wake
Sleep Onset NREM
REM
State of Consciousness
Types of IVs and DVs
Example of Study Using Quantitative IV
Question: Do different dosage levels of a drug
affect performance of people with ADHD?
Result: Increased
dosages lead to
improved performance
only up to a point.
Percentage Correct
Method: Randomly assign participants to
90
conditions and assess
80
70
performance on
60
worksheets.
50
40
30
20
10
0
Placebo
10 mg
20 mg
Dosage of Methylphenidate
30 mg
Types of IVs
Types of Independent Variables
Task Variable—An IV whose different conditions
involve differences in a task that participants
perform
Instructional Variable—An IV whose different
conditions involve different instructions given
by the researcher to the participants
Situational Variable—An IV whose different
conditions involve different environments of
contexts for different participants
Controversy: Studying Race
• Researchers often create quasi-experimental
independent variables according to race of the
participant. Is this an appropriate method?
– Categorization to groups is often unclear in research
and poorly done (e.g., based on a participant’s name)
– Scientists increasingly believe that race is a social, not
a biological, construct
– Cultural factors as a “cause” of behavior may be more
significant than biological factors
– Social conditions may influence behaviors greatly
– Research reports often do not report the
race/ethnicity of the investigator, a possible source of
biosocial effects
Controversy: Studying Depression
Experimentally
• Are experiments on depression ethical?
– Can an experimenter ethically induce depression?
– Is it ethical to depart from effective treatments to
study a person with depression?
– Can you realistically induce depression in a
nondepressed sample?
• Researchers have developed techniques to
induce short-term, reversible states
resembling very mild depression
• Experiments on depression can be safe and
effective.
Comparing Two Groups
• Independent Groups Designs—Sometimes it is
useful to create two separate groups with
different people in each group.
• Repeated Measures Designs—Sometimes it is
effective and efficient to test the same people
in two conditions, so you need only half as
many people as in independent groups
designs.
Comparing Two Groups
• Generalizing Research Results
– If we study college students in an experiment, can
we generalize our results to other groups?
• If we study rats, fish, and pigeons, can we
generalize the results to people?
– When female fish saw male fish in the presence of
other female fish, that male fish became more
attractive as evidenced by the female’s interest.
– Does that behavior pattern hold for women seeing
a man in the presence of other women? The
answer seems to be yes.
Comparing Multiple Groups
• Sometimes two groups are not enough to give
us the answers we are looking for
• Researchers use multiple group designs
Comparing Multiple Groups
Example of a Multiple-Group Design
Rating of Cartoons
Question: Do people rate jokes differently if they expect
the jokes to be funny or not funny?
Method: Randomly assign participants to groups who
expect funny jokes, unfunny jokes, or who have no
particular expectations.
5
3.9
4
3.6
Have participants
3
3
rate the jokes
2
Results: Expectations
1
affected ratings
0
Not Very Funny
Neutral
Very Funny
Type of Priming Message
Data Analysis
• Experiments with Two Groups
– Student’s t test for independent groups or
Student’s t test for repeated measures groups
• Experiments with Multiple Groups
• Analysis of Variance (ANOVA)
– Planned comparisons: Comparisons between
groups in a multiple-group study; comparisons are
planned before doing the data analysis
– Post hoc comparisons: Comparisons between
groups in a multiple-group study; comparisons are
not determined before the ANOVA
Note: ANOVA can be used with two groups, but typically is
not if a study involves only one IV with two groups