true experimental research design

1. Addis Ababa University
Collage of Education and Behavioral Studies
Department of Science and Mathematics
Education
True Experimental Research Design
BY
Aweke Shishigu ( PhD Candidate)
November 2014, Addis Ababa
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2. True Experimental Research Design
True experimental design is the most accurate form of
experimental research.
For an experiment to be classified as a true
experimental design, it must fit all of the following
criteria:
1. Random selection of subjects from a population
2. Random assignment of subjects to either control or
experimental group
3. One or more intervention (treatment) for the experimental
group
4. Non- contamination of experimental and control group.
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3. Dependent and Independent Variable
Independent variable: The variable that is
systematically controlled by the researcher to
determine its effect.
Dependent variable: The outcome variable or the
one you are simply measuring.
The independent variable is the cause and the
dependent variable is the effect.
The independent variable always happens first and
the dependent variable always happens second.
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4. Experimental and Control Group
Experimental group: The group that receives some
kind of change (treatment) to their natural
environment.
Control group: the group that does not receive a
treatment.
In a true experiment, every participant is exposed
to the same environment including the
characteristics of the room, the experimenters,
and the instructions the participants receive.
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5. Types of True Experimental Design
1. Two group post test only design
2. Pretest Post Test Control Group Design
3. Matched control group design
4. Solomon four group design
5. Factorial Design
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6. Two group post test only design
1. Two group post test only design:
 Notation:
X- treatment or intervention
O- observation or measurement of the dependent variable
R - random assignment
Group 1: R X O1
Group 2: R O2
o Disadvantage of this design: Absence of pre test:
because without a pretest measure, a researcher
cannot be sure that participants in the control and
experimental groups were equivalent initially.
• It is used only when Pre testing is not possible.
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7. 2. Pretest Post Test Control Group Design
o A pretest and posttest be administered to both the
control and experimental group.
o The two groups should be equivalent at beginning
o Observed differences must result from the
treatment
Group 1: R O1 X O2
Group 2: R O3 O4
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8. 3. Matched control group design
o After participants have been randomly selected from a population,
they are rank ordered according to a specific variable closely related
to the posttest measure.
Group 1: R X O1
Matching of participants
Group 2: R O2
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9. 4. Solomon four group design
o There are four groups: two experimental and two
control groups
o It Controls most threats to the internal validity of
an experiment.
Group 1- R O1 X O2
Group 2- R O3 O4
Group 3- R X O5
Group 4- R O6
o If O2 and O5 are not different from one another and they are
significantly different from O1, O3, O4, and O6 (and O1, O3,
O4, and O6 are not different from one another), then the
difference can be attributed to the treatment (independent
variable)
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10. 5. Factorial Design
o It is the design in which two or more independent variables
are simultaneously studied .
o The main effect and interaction effect of two or more
variables are studied.
o A factor is a discrete variable used to classify experimental
units.
Example
1. Gender might be a factor with two levels male and female
2. Diet might be a factor with three levels low, medium” and
high protein.
Remark: The levels within each factor can be discrete, such as
Drug A and Drug B, or they may be quantitative such as 0,
10, 20 and 30 mg/kg.
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11. Classification of Factorial Design
o Factorial designs are classified by the number of
levels of each factor and the number of factors.
o 2k Factorial design consists of k factors each at
two levels and 2k experimental conditions
Example: 32 design contains 2 factors each with 3
levels and 32 =9 experimental conditions.
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12. 22 Factorial design
o It contains two factors each at two level and 4
experimental conditions
a0 a1
b0
b1
12
40 50
30
60

13. Treatment combinations
1 a0b0 00
a a1b0 10
b a0b1 01
ab a1b1 11
o simple effect of A at b0 =50-40=10
o simple effect of A at b1 = 60-30= 30
o simple effect of B at a0 = 30-40= -10
o simple effect of B at a1 = 60-50= 10
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a0 a1
40 50 b0
30 60 b1

14. Cnd…
o Main effect of A= the average of simple effects
=10+30/2=20
o Main effect of B= the average of simple effects
=-10+10/2=0
Interaction= (difference b/n simple effects)/2
= 30-10/2=10
Or =10-(10)/2=10
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15. Internal and External Validity
Internal validity: The accuracy of the outcome of
an experiment –Asks the question; is the outcome
really due to the independent variable
(treatment)?
External validity: The extent to which the results
of an experiment can be generalized.
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16. Threats to Internal Validity
 History: Refers to events other than the treatment that occur during
the experiment which may influence the outcome.
Solution: Make sure that all participants are from similar area (from the same
population)
 Maturation: changes due to natural development such as physical,
mental, emotional, or spiritual.
Solution: Both groups experience the same developmental processes hence
maturation has no effect on your experiment. It is a threat for one group design
 Selection: Biased selection of participants: were the subjects self-selected
into experimental and control groups, which could affect the
dependent variable?
Solution: assign subjects by random selection and ensure that all had equal
chance of getting the experiment or control condition.
 Testing: Sensitization due to pretest on the the experimental groups
Solution: use non-reactive measures when possible.
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17. Instrumentation: Biases due to testing procedures
Solution: Do not change the measurement method and
instruments
Regression: The tendency for extreme scorers to
move toward more typical performance when retested
Solution: Make sure that subjects were equivalent at the
beginning of the experiment.
Mortality: Changes in group composition due to drop
out of some participants from the study
Solution: Randomly drop the same number of subjects from
the other group
Design Contamination: The control group find out
or aware about the experimental treatment.
Solution: Do not mix the control and experimental groups
until your research is completed.
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18. Threats to External Validity
Multiple treatment effect: Several
treatments occur simultaneously
Reactive arrangement: testing
procedures
Pretest Sensitization: Subjects who
take a pre-test are sensitized to the
treatment which is to follow
Experimenter effect: effect due to the
presence of the experiment
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19. *Thank you*
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