Physical Variables
Social Variables
Personality Variables
Context Variables
What are physical variables?
Physical variables are aspects of the testing
situation that need to be controlled:
 day of the week
 experimental room
 lighting
Physical Variables
Explain elimination.
Elimination completely removes extraneous
physical variables from the experimental
situation (e.g., soundproofing a room).
Removal of extraneous physical variables
prevents them from operating differently across
different treatment conditions.
Physical Variables
How does constancy of conditions work?
Constancy of conditions controls extraneous
physical variables by keeping all aspects of the
treatment conditions identical, except for the
independent variable.
For example, test all subjects in the same room
or at the same time of day.
Physical Variables
How does balancing work?
Balancing controls extraneous physical
variables by equally distributing their effects
across treatment conditions.
For example, running half of the subjects
in each condition in the morning and half in
the evening.
Physical Variables
In which order should you use these techniques?
1. Eliminate extraneous variables whenever
possible.
2. Keep conditions constant where elimination
is not possible.
3. Balance the effects of extraneous variables
when constancy of conditions is not possible.
Physical Variables
What are social variables?
Social variables are aspects of the relationships
between subjects and experimenters that can
influence experimental results.
These include demand characteristics and
experimenter bias.
Social Variables
Explain demand characteristics.
Demand characteristics are cues within the
experimental situation that demand or elicit
specific participant responses.
Example: students cue professors to wrap up
their lectures by packing their binders, books,
and water bottles, and by looking at the door.
Social Variables
How can demand characteristics threaten internal
validity?
Demand characteristics can confound an
experiment if they vary across experimental
conditions.
Subjects may act to confirm what they think
is the experimental hypothesis.
Social Variables
What is a single-blind experiment?
In a single-blind experiment, subjects are
not told their treatment condition.
For example, in a single-blind drug study,
the experimental and control groups might
receive capsules that look and taste identical.
Social Variables
How do single-blind experiments control demand
characteristics?
When subjects are not told their treatment
condition, this eliminates cues that might
alter their behavior.
Social Variables
What is the placebo effect?
The placebo effect is when a subject receives
an inert treatment and improves because of
positive expectancies.
Social Variables
How do cover stories control demand
characteristics?
A cover story is a false plausible explanation
of the experimental procedures to disguise the
research hypothesis from the subjects.
They should be used sparingly, since they are
a form of deception.
Social Variables
What is experimenter bias?
Experimenter bias is any behavior by the
experimenter that can confound the experiment.
For example, an experimenter might provide
more attention to subjects in one condition than
another.
Social Variables
What is the Rosenthal effect?
The Rosenthal effect is the phenomenon in
which experimenters treat subjects differently
based on their expectations and their resulting
actions influence subject performance.
This is also called the Pygmalion effect and
self-fulfilling prophecy.
Social Variables
What is the Rosenthal effect?
For example, teachers might give more
attention and feedback to high aptitude
students than to low aptitude students.
The Rosenthal effect can confound an
experiment, producing results consistent
with the experimenter’s expectations.
Social Variables
Why is a double-blind design superior to a singleblind design in controlling experimenter bias?
Single-blind experiments only control demand
characteristics, since subjects are blinded to
their condition.
Double-blind experiments control both demand
characteristics and experimenter bias, since both
the experimenter and subjects are blinded.
Social Variables
How might an experimenter's personality affect
experimental results?
Research on experimenter personality shows
that when experimenters are warm and friendly,
subjects learn more, talk more, earn better test
scores, and are eager to please.
Hostile or authoritarian experimenters obtain
inferior subject performance.
Personality Variables
How can experimenters control personality
variables?
Employ multiple experimenters to run an equal
number of subjects in each of the experimental
conditions (balancing).
Treat “experimenter” as an independent variable
in statistical analysis. If an interaction is found,
then the experiment was confounded.
Personality Variables
How can experimenters control personality
variables?
When there is a single experimenter, minimize
face-to-face contact and closely follow the script.
Videotape sessions to confirm consistent
performance.
Personality Variables
How do volunteers differ from nonvolunteers?
Volunteers are more sociable, score higher
in social desirability, hold more liberal social
and political attitudes, are less authoritarian,
and score higher on intelligence tests than
nonvolunteers.
Personality Variables
What are context variables?
Context variables are extraneous variables
produced by experimental procedures created
by the research setting environment, like
assignment of participants to conditions.
Context Variables
When might subjects select the experiment?
When we allow subjects to sign up for
experiments whose titles differ in their appeal:
“The Memory Test Experiment”
“The Heavy Metal Music Experiment”
However, this could result in a biased sample
threatening external validity.
Context Variables
Why shouldn’t you run your friends in your
experiment?
Selecting your friends might bias your sample,
threatening external validity.
Both you and your friends might act differently
in your experiment than strangers.
Context Variables
Summarize the folklore about subjects.
Subjects who sign up late in the semester may
be less motivated and may behave differently
than those who sign up earlier in the semester.
Rosenthal speculated that the differences seen
at the start and end of an experiment may be
just as likely due to changes in the experimenter.
Context Variables