Introduction to
Experimental Research
Designs
Why Psychologists Conduct Experiments?
► To
test
► hypotheses
derived from theories
► effectiveness
► The
of treatments and programs
goal of experimental research
► explanation
►examine
the causes of behavior
What is an Experiment?
►
Research method in which we examine
► the
effect of independent variable on the dependent
variable,
► where the independent variable is manipulated through
treatment or intervention(s), &
► the effect of those interventions is observed on the
dependent variable.
►
4 things to keep in mind
► Independent
variable & dependent variable
► Experimental group & control group
► Pre-testing & Post-testing
► Random selection & random assignment
Some Definitions
►
►
Dependent Variable
►
Criterion by which the results of the experiment are judged.
►
Variable that is expected to be dependent on the manipulation of the independent
variable
Independent Variable
►
►
Experimental Group
►
►
Any variable that can be manipulated, or altered, independently of any other
variable
Group of subjects exposed to the experimental treatment
Control Group
►
Group of subjects exposed to the control condition
►
Not exposed to the experimental treatment
Definitions
► Random
Selection & Random Assignment
► Random
selection means that every subject has an
equal chance of being selected.
► Random
assignment means that every subject has
an equal chance of being assigned to experimental
or control group.
► Pre-testing
& Post-testing
Characteristics of a True Experiment
►A
true experimental research design must
essentially consist of the following three
characteristics:
1.
Manipulation
2.
Control
3.
Randomization
Control
Control is the essential ingredient of experiments
► control is gained through
► manipulation, (amount of IV)
► holding conditions constant, (avoiding confoundings)
► and balancing (similar compareable groups (RA))
► With proper use of control techniques, an experiment
has internal validity (the extent to which a study establishes a
►
trustworthy cause-and-effect relationship between a treatment and
an outcome)
Internal validity
► internal
validity is established by three conditions
required for causal inference:
► covariation,
► time-order
(when one changes, the other must also change.)
relationship, (The presumed cause (hitting your head) must
occur before the presumed effect)
► elimination
of plausible alternative causes
(you would
have to consider and rule out other possible causes of your presumed effect)
Zechmeister, E. B., Zechmeister, J. S., & Shaughnessy, J. J. (2015). Research methods in psychology.
McGraw-Hill Higher Education.
RANDOM GROUPS DESIGN
►
►
►
In an independent groups design, each group of subjects
participates in only one condition of the independent
variable.
Random assignment to conditions is used to form
comparable groups by balancing or averaging subject
characteristics (individual differences) across the conditions
of the independent variable manipulation.
When random assignment is used to form independent
groups for the levels of the independent variable, the
experiment is called a random groups design.
An Example of a Random Groups
Design
►
Girls dissatisfaction with their body, conducted in
the United Kingdom by Dittmar, Halliwell, and Ive (2006).
Exposure to very
thin body images
causes
young girls to
experience
negative feelings
Holding Conditions
Constant
Small groups of young girls (5 1⁄2–6 1⁄2 years old) were
read a story about “Mira” as she went shopping for
clothes and prepared to go to a
birthday party. Along with a picture books with
six scenes
Random Assignment
Dependent
Variable
Images of Emme
Child Figure Rating
Scale
Perceived Actual
Body Shape
Body
dissatisfaction
Balancing
Ideal Body
Shape
No significant
Body
dissatisfaction
Covariation
windows of clothes
shops, colorful balloons
Holding Conditions
Constant
Time order Relationship
Images of
Barbie
Independent
Variable,
manipulated
No significant
Body
dissatisfaction
Thank you