This document discusses experimental research design. It describes independent group design, where separate groups represent different conditions of the independent variable. Random assignment of subjects to conditions is used. The document also discusses internal validity, the logic of experimental research including manipulation and control of variables, and threats to internal validity such as alternative explanations. As an example, it summarizes an experiment that examined how exposure to thin images affected girls' body satisfaction by randomly assigning girls to view photos of either thin, average, or plus-sized models.

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2. Independentgroup
design
ShayanIqbal

3. Matched
Group
Design
Independent Group
Design
Random Group
Design
Natural group
design

4. Important Concepts Related To Experimental
Research
Why psychologists conduct experiments ?
• Researcher conduct experiment to test
the hypothesis about causes and
behavior.
• Experiments allow researcher to decide
whether a treatment or program
effectively change behavior.
Random Group Design
• When random assignment to treatment
conditions is used, the independent group
design is called a random group design.

5. Important Concepts Related To Experimental
Research
Random Assignment
• Experiments in which each
separate group of subject in the
experiment represent a
different conditions as defined
by level of IV.
• It uses random assignment of
subject to conditions in order to
prior to implementing the IV.
Rationale of Independent group
design
• Can conduct the experiment to
study treatment effect.
• Ability to make between group
comparison.
• Use of control group to infer
change in experimental group
only due to treatment.
Threats to internal validity
• Occurrence of event between
treatment condition and
measurement effect.
• Time span between treatment
conditions and measurement
effects.
• Presence of plausible
alternative.

6. Logic of experimental research
Experimental manipulation
• Researcher manipulate an IV in
an experiment to observe the
effect on behavior, as assessed
by the dependent variable.
Experimental control
• Experimental control allow
researcher to make causal
inference that the IV cause the
effect change in DV.
• Manipulation, holding condition
constant, balancing.
Internal validity
• The IV caused the difference in
behavior as measured by the DV.
• Three conditions required for
causal inferences; covariation,
time order relationship,
elimination of plausible
alternative causes.

7. High internal validity of experiment
Covariation
• When to different measures of
the same people , event vary to
together- that is when scores on
one variable covary with scores
on other variable.
Time order relationship
• The efforts to maintain
minimum possible time
between treatments and
measurement effects.
Elimination of plausible
alternative causes
• When confounding occur
plausible alternative explanation
for the observed covariation
exists, the experiment lacks
internal validity.
• Holding condition constant and
balancing.

8. Elimination of plausible alternative causes
Holding condition constant
• It is to make sure that the IV is
the only factor that differs
systematically across the group.
Balancing
• Balancing the difference
among subject tested from
heterogenous population.
• Forming comparable groups at
the start of experiment.
Averaging
• As a part of balancing the
researcher averages the
individual differences among
subject across the group of the
experiment .

9. Elimination of plausible alternative
Objective
• Dittmar et al.(2006).
• There goal was to determine
whether exposure to very thin
body images causes young girls
experience negative experience
about their own body.
Sample
• In the experiment small group
of young girls read a story
about Mira as she went
shopping for clothes and
prepared to go to the birthday
party.
Procedure
• as they heard the story, the girls
looked at the pictures with six
scenes related to story.

10. Treatment conditions
Condition 1 Condition 2
Condition 3

11. Conditions of experimental design
IV and DV Manipulation of IV Cause effect b/w IV and DV
IV: Exposure to Body
Image
DV: Dissatisfaction
IV: Exposure to Body Image
C1 C2 C3
X1 X2 X3
IV: Exposure to Body Image
Barbi Eme NU
O3
O2
O1

12. Random Assignment to treatment conditions and
observation of behavior
Stage 1 : Random Assignment
Stage 2: Random assignment to
treatment condition
Stage 3: Observation of behavior
Girls
R1 R2 R3
X1 X2 X3
Barbi Eme NU
O3
O2
O1
R1
n=17
R2
n=20
R3
n=20
R1 R2 R3

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