1
Chapter 8. Experiments
Chapter 8. Experiments
Experiments usually involve two stages
Experiments usually involve two stages
Taking action
Taking action
Observing the consequences of that action.
Observing the consequences of that action.
Topics covered in this chapter
Topics covered in this chapter
Topics appropriate for experimental research
Topics appropriate for experimental research
The classical experimental design
The classical experimental design
Variation of experimental design: Pre
Variation of experimental design: Pre-
-experimental designs
experimental designs
Internal and external validity issues
Internal and external validity issues
Strengths and weaknesses
Strengths and weaknesses
Topics Appropriate for
Topics Appropriate for
Experimental Research
Experimental Research
Hypothesis testing involving limited and well
Hypothesis testing involving limited and well-
-
defined concepts and propositions.
defined concepts and propositions.
Explanatory purposes rather than descriptive
Explanatory purposes rather than descriptive
purposes
purposes
Laboratory or natural experiments
Laboratory or natural experiments
The Classical Experimental Design
The Classical Experimental Design
Purpose
Purpose
Examine the effect of an experiment stimulus (the
Examine the effect of an experiment stimulus (the
independent variable) on a dependent variable.
independent variable) on a dependent variable.
Procedure
Procedure
Step 1. Randomly assign subjects in your sample to an
Step 1. Randomly assign subjects in your sample to an
experimental group and a control group.
experimental group and a control group.
Step 2. Pretest two groups to make sure they are similar in
Step 2. Pretest two groups to make sure they are similar in
ways related to your experiment.
ways related to your experiment.
Step 3. The experimental group is exposed to the
Step 3. The experimental group is exposed to the
experimental stimulus, but not the control group.
experimental stimulus, but not the control group.
Step 4. Compare whether the experiment group behave
Step 4. Compare whether the experiment group behave
differently from the control group after the experiment.
differently from the control group after the experiment.
An Example
An Example
Harvard Test of Inflected Acquisition (HTIA)
Harvard Test of Inflected Acquisition (HTIA)
(Rosenthal & Jacobson, 1968)
(Rosenthal & Jacobson, 1968)
To test the theory that the way others perceive us is largely
To test the theory that the way others perceive us is largely
conditioned by expectations they may have in advance.
conditioned by expectations they may have in advance.
Process
Process
The HTIA test was administered to a group of students
The HTIA test was administered to a group of students
Half of the subjects were randomly selected to be in the
Half of the subjects were randomly selected to be in the
experimental group, the other half in the control group
experimental group, the other half in the control group
The teachers were told that those students in the experimental
The teachers were told that those students in the experimental
group are very likely to exhibit a sudden spurt in academic abil
group are very likely to exhibit a sudden spurt in academic abilities
ities
during the coming year based on their score on the test.
during the coming year based on their score on the test.
Dependent variable
Dependent variable
Academic performance in the coming year (another real test)
Academic performance in the coming year (another real test)
Independent variable
Independent variable
Other’s expectation (whether a student was described to the teac
Other’s expectation (whether a student was described to the teachers
hers
as being likely to exhibit a sudden spurt soon)
as being likely to exhibit a sudden spurt soon)
An example
An example -
-continued
continued
Result:
Result:
In the following year, when the researchers went back to look at
In the following year, when the researchers went back to look at
academic performance of the students, they found that the studen
academic performance of the students, they found that the students
ts
in the experimental group far exceeded those in the control grou
in the experimental group far exceeded those in the control group
p
Because teachers gave students the grades, it stands to reason t
Because teachers gave students the grades, it stands to reason that
hat
teachers gave higher grades to those students in the experimenta
teachers gave higher grades to those students in the experimental
l
groups because researchers told them those students were likely
groups because researchers told them those students were likely to
to
exhibit a sudden spurt in academic abilities.
exhibit a sudden spurt in academic abilities.
Note that the HTIA test is a fake. It was used only to build
Note that the HTIA test is a fake. It was used only to build
up teachers’ expectations. Whether somebody is in the
up teachers’ expectations. Whether somebody is in the
experimental group or not is totally random and has
experimental group or not is totally random and has
nothing to be with the HTIA test scores.
nothing to be with the HTIA test scores.
What if one cannot use classical
What if one cannot use classical
experimental design due to
experimental design due to
circumstance?
circumstance? -
-Three pre
Three pre-
-
experimental designs.
experimental designs.
One
One-
-shot case study
shot case study
One
One-
-group pre
group pre-
-test and post
test and post-
-test design
test design
Static
Static-
-group comparison
group comparison
