Independent and Dependent Variables
Operational Definitions
Evaluating Operational Definitions
Planning the Method Section
What is an independent variable?
An independent variable (IV) is the variable
(antecedent condition) an experimenter
intentionally manipulates.
Levels of an independent variable are the
values of the IV created by the experimenter.
An experiment requires at least two levels.
Independent and Dependent Variables
Explain confounding.
An experiment is confounded when the value
of an extraneous variable systematically
changes along with the independent variable.
For example, we could confound our experiment
if we ran experimental subjects in the morning
and control subjects at night.
Independent and Dependent Variables
What is a dependent variable?
A dependent variable is the outcome measure
the experimenter uses to assess the change in
behavior produced by the independent variable.
The dependent variable depends on the value
of the independent variable.
Independent and Dependent Variables
What is an operational definition?
An operational definition specifies the exact
meaning of a variable in an experiment by
defining it in terms of observable operations,
procedures, and measurements.
Operational Definitions
What is an operational definition?
An experimental operational definition
specifies the exact procedure for creating
values of the independent variable.
A measured operational definition specifies
the exact procedure for measuring the
dependent variable.
Operational Definitions
What are the properties of a nominal scale?
A nominal scale assigns items to two or more
distinct categories that can be named using a
shared feature, but does not measure their
magnitude.
Example: you can sort canines into friendly and
shy categories.
Evaluating Operational Definitions
What are the properties of an ordinal scale?
An ordinal scale measures the magnitude of
the dependent variable using ranks, but does
not assign precise values.
This scale allows us to make statements about
relative speed, but not precise speed, like a
runner’s place in a marathon.
Evaluating Operational Definitions
What are the properties of an interval scale?
An interval scale measures the magnitude
of the dependent variable using equal intervals
between values with no absolute zero point.
Example: degrees Celsius or Fahrenheit and
Sarnoff and Zimbardo’s (1961) 0-100 scale.
Evaluating Operational Definitions
What are the properties of a ratio scale?
A ratio scale measures the magnitude of the
dependent variable using equal intervals
between values and an absolute zero.
This scale allows us to state that 2 meters are
twice as long as 1 meter.
Example: distance in meters or time in seconds.
Evaluating Operational Definitions
What does reliability mean?
Reliability refers to the consistency of
experimental operational definitions and
measured operational definitions.
Example: a reliable bathroom scale should
display the same weight if you measure
yourself three times in the same minute.
Evaluating Operational Definitions
Explain interrater reliability.
Interrater reliability is the degree to which
observers agree in their measurement
of the behavior.
Example: the degree to which three observers
agree when scoring the same personal essays
for optimism.
Evaluating Operational Definitions
Explain test-retest reliability.
Test-retest reliability means the degree to
which a person's scores are consistent across
two or more administrations of a measurement
procedure.
Example: highly correlated scores on the
Wechsler Adult Intelligence Scale-Revised
when it is administered twice, 2 weeks apart.
Evaluating Operational Definitions
Explain interitem reliability.
Interitem reliability measures the degree to
which different parts of an instrument
(questionnaire or test) that are designed to
measure the same variable achieve consistent
results.
Evaluating Operational Definitions
What does validity mean?
Validity means the operational definition
accurately manipulates the independent
variable or measures the dependent variable.
Evaluating Operational Definitions
What is face validity?
Face validity is the degree to which the validity
of a manipulation or measurement technique is
self-evident. This is the least stringent form of
validity.
For example, using a ruler to measure pupil size.
Evaluating Operational Definitions
What is content validity?
Content validity means how accurately a
measurement procedure samples the content
of the dependent variable.
Example: an exam over chapters 1-4 that only
contains questions about chapter 2 has poor
content validity.
Evaluating Operational Definitions
What is predictive validity?
Predictive validity means how accurately a
measurement procedure predicts future
performance.
Example: the ACT has predictive validity if these
scores are significantly correlated with college
GPA.
Evaluating Operational Definitions
What is construct validity?
Construct validity is how accurately an
operational definition represents a construct.
Example: a construct of abusive parents might
include their perception of their neighbors as
unfriendly.
Evaluating Operational Definitions
Explain internal validity.
Internal validity is the degree to which changes
in the dependent variable across treatment
conditions were due to the independent variable.
Internal validity establishes a cause-and-effect
relationship between the independent and
dependent variables.
Evaluating Operational Definitions
Explain the problem of confounding.
Confounding occurs when an extraneous
variable systematically changes across the
experimental conditions.
Example: a study comparing the effects of
meditation and prayer on blood pressure would
be confounded if one group exercised more.
Evaluating Operational Definitions
Explain history threat.
History threat occurs when an event outside
the experiment threatens internal validity by
changing the dependent variable.
Example: subjects in group A were weighed
before lunch while those in group B were
weighed after lunch.
Evaluating Operational Definitions
Explain maturation threat.
Maturation threat is produced when physical
or psychological changes in the subject threaten
internal validity by changing the DV.
Example: boredom may increase subject errors
on a proofing task (DV).
Evaluating Operational Definitions
Explain testing threat.
Testing threat occurs when prior exposure to a
measurement procedure affects performance on
this measure during the experiment.
Example: experimental subjects used a blood
pressure cuff daily, while control subjects only
used one during a pretest measurement.
Evaluating Operational Definitions
Explain instrumentation threat.
Instrumentation threat is when changes in
the measurement instrument or measuring
procedure threatens internal validity.
Example: if reaction time measurements
became less accurate during the experimental
than the control conditions.
Evaluating Operational Definitions
Explain statistical regression threat.
Statistical regression threat occurs when
subjects are assigned to conditions on the basis
of extreme scores, the measurement procedure
is not completely reliable, and subjects are
retested using the same procedure to measure
change on the dependent variable.
Evaluating Operational Definitions
Explain selection threat.
Selection threat occurs when individual
differences are not balanced across treatment
conditions by the assignment procedure.
Example: despite random assignment, subjects
in the experimental group were more extroverted
than those in the control group.
Evaluating Operational Definitions
Explain subject mortality threat.
Subject mortality threat occurs when subjects
drop out of experimental conditions at different
rates.
Example: even if subjects in each group started
out with comparable anxiety scores, drop out
could produce differences on this variable.
Evaluating Operational Definitions
Explain selection interactions.
Selection interactions occur when a selection
threat combines with at least one other threat
(history, maturation, statistical regression,
subject mortality, or testing).
Evaluating Operational Definitions
What is the purpose of the Method section of an
APA report?
The Method section of an APA research report
describes the Participants, Apparatus or
Materials, and Procedure of the experiment.
This section provides the reader with sufficient
detail (who, what, when, and how) to exactly
replicate your study.
Planning the Method Section
When is an Apparatus section needed?
An Apparatus section of an APA research
report is appropriate when the equipment used
in a study was unique or specialized, or when we
need to explain the capabilities of more common
equipment so that the reader can better evaluate
or replicate the experiment.
Planning the Method Section