Non-Experimental Design

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Day 6: Non-Experimental &
Experimental Design
Where are the beakers??
What kind of research is considered
the “gold standard” by the Institute of
Education Sciences?
A.
B.
C.
D.
Descriptive
Causal-Comparative
Correlational
Experimental
Why?
Why does most educational
research use non-experimental
designs?
What is the purpose of
non-experimental designs?
Causal-Comparative Example
Green & Jaquess (1987)
– Interested in the effect of high school
students’ part-time employment on their
academic achievement.
– Sample: 477 high school juniors who were
unemployed or employed > 10 hours/wk.
Causal-Comparative Design
A study in which the researcher attempts to
determine the cause, or reason, for preexisting differences in groups of individuals
At least two different groups are compared on
a dependent variable or measure of
performance (called the “effect”) because the
independent variable (called the “cause”) has
already occurred or cannot be manipulated
Causal-Comparative Design
Ex-post facto
– Causes studied after they have exerted
their effect on another variable.
Causal-Comparative Design
Drawbacks
– Difficult to establish causality based on
collected data.
– Unmeasured variables (confounding
variables) are always a source of potential
alternative causal explanations.
Some Thought Questions…
Correlational Design
Determines whether and to what degree
a relationship exists between two or
more quantifiable variables.
Correlational Design
The degree of the relationship is
expressed as a coefficient of correlation
Examples
– Relationship between math achievement
and math attitude
– Relationship between degree of a school’s
racial diversity and student use of
stereotypical language
– Your topics?
Correlation coefficient…
-1.00
strong negative
0.00
+1.00
strong positive
no
relationship
Advantages of Correlational Design
Analysis of relationships among a large
number of variables in a single study
Information about the degree of the
relationship between the variables being
studied
Cautions
A relationship between two variables
does not mean one causes the other
(Think about the reading achievement
and body weight correlations)
Possibility of low reliability of the
instruments makes it difficult to identify
relationships
Cautions
Lack of variability in scores (e.g.
everyone scoring very, very low;
everyone scoring very, very high; etc.)
makes it difficult to identify relationships
Large sample sizes and/or using many
variables can identify significant
relationships for statistical reasons and
not because the relationships really exist
(Avoid shotgun approach)
Cautions
Need to identify your sample to know
what is actually being compared.
If using predictor variables, time interval
between collecting the predictor and
criterion variable data is important.
Correlational Designs
Guidelines for interpreting the size of
correlation coefficients
– Much larger correlations are needed for
predictions with individuals than with groups
Crude group predictions can be made with
correlations as low as .40 to .60
Predictions for individuals require
correlations above .75
Correlational Designs
Guidelines for interpreting the size of
correlation coefficients
– Exploratory studies
Correlations of .25 to .40 indicate the need
for further research
Much higher correlations are needed to
confirm or test hypotheses
Correlational Designs
Criteria for evaluating correlational studies
– Causation should not be inferred from
correlational studies
– Practical significance should not be confused
with statistical significance
– The size of the correlation should be
sufficient for the use of the results
(individuals vs groups)
Think…
If you were going to take your action
research topic, and create a causalcomparative study, what would it look
like?
--OR-If you were going to take your action
research project, and create a
correlational study, what would it look
like?
Experimental Design
The Gold Standard?
To Review
Why is most educational research
comprised of non-experimental research
designs?
To Review
What is the purpose of non-experimental
research?
To Review
How does the independent variable
function in non-experimental research?
To Review
Can non-experimental research claim
causality?
An example
Read the example given in class and in
pairs respond to the questions
Experimental Research
Purpose
– To make causal inferences about the relationship
between the independent and dependent variables
Characteristics
– Direct manipulation of the independent variable
– Control of extraneous variables
Experimental Designs
Single Group Post-test
Single Group Pre-test Post-test
Non-Equivalent Groups Post-test
Quasi-Experimental Design
Randomized Post-test only
Randomized Pre-test Post-test
Factorial
Examples
Experimental Validity
Internal validity
– The extent to which the independent
variable, and not other extraneous
variables , produced the observed effect on
the dependent variable
External validity
– The extent to which the results are
generalizable
Internal Validity
Threats that reduce the level of confidence in
any causal conclusions
Key Question: Is this a plausible threat to the
internal validity of the study?
Threats to Internal Validity
History
– Extraneous events have an effect on the subjects’
performance on the dependent variable
– Ex - The crash of the stock market, 9-11, the
invasion of Iraq, etc.
Selection
– Groups that are initially not equal due to
differences in the subjects in those groups
– Ex - Positive and negative attitudes, high and low
achievers, etc.
Threats to Internal Validity
Maturation
– Changes experienced within the subject over
time
Pretesting
– The effect of having taken a pretest
Instrumentation
– Poor technical quality (i.e. validity, reliability)
or changes in instrumentation
Threats to Internal Validity
Subject attrition
– Differential loss of subjects from groups
Statistical regression
– The natural movement of extreme scores toward the
mean
Diffusion of treatment
– The treatment is given to the control group
Experimenter effects
– Different characteristics or expectations of those
implementing the treatments across groups
Threats to Internal Validity
Subject effects
– The effects of being aware that one is
involved in a study
– Four types
Hawthorne effect
John Henry effect
Resentful demoralization
Novelty effect
Internal Validity
Key Point: Ultimately, validity is a matter
of judgment. Ask if it is reasonable that
possible threats are likely to affect the
results.
External Validity
The extent to which results can be
generalized from a sample to a
particular population.
Question – Why would really good
internal validity often result in poor
external validity?
External Validity
Factors affecting external validity
– Subjects
Representativeness of the sample in
comparison to the population
Personal characteristics of the subjects
– Situations - characteristics of the setting
Specific environment
Special situation
Particular school
External Validity
Importance of explanation of sampling
procedures
Experimental Designs
Single Group Post-test
Single Group Pre-test Post-test – Libby, Deb
Non-Equivalent Groups Post-test – Mary, Cheryl
Quasi-Experimental Design – Pete, Laura
Randomized Post-test only – Amanda, Nicole, Tam
Randomized Pre-test Post-test – Karen, Jen, Justin
Examples
Your Task
Based on the topic of your proposal,
design an experimental study using the
design you were assigned.
– Write a research question and hypothesis.
– Sketch out the methods.
Identify strengths and weaknesses of
each design.
Experimental Designs
Notation
– R indicates random selection or random
assignment
– O indicates an observation
Test
Observation score
Scale score
– X indicates a treatment
– A, B, C, ... indicates a group
Pre-Experimental Designs
No pre-experimental design controls internal
validity threats well
Single group pretest only
–A X O
– Internal validity threats
History, maturation, attrition, experimenter effects, subject
effects, and instrumentation are viable threats
Useful only when the research is sure of the status of the
knowledge, skill, or attitude being changed and there are
no extraneous variables affecting the results
Pre-Experimental Designs
Single group pretest post-test
– AOXO
– Internal validity threats
Maturation and pretesting are threats
History and instrumentation are potential threats
– Useful when subject effects will not influence the
results, history effects can be minimized, and
multiple pretests and post-tests are used
Pre-Experimental Designs
Non-equivalent groups post-test only
–A X O
B
O
– Internal validity threats
Definite Threat: Selection
Potential Threats: History, maturation, and
instrumentation
– Useful when groups are comparable and subjects
can be assumed to be about the same at the
beginning of the study
Quasi-Experimental Designs
Types
– Non-equivalent pretest/post-test, experimental
control groups
AOXO
BO O
– Non-equivalent pretest/post-test, multiple treatment
groups
A O X1 O
B O X2 O
Useful when subjects are in pre-existing
groups (e.g. classes, schools, teams, etc.)
Quasi-Experimental Designs
Threats to internal validity
– Selection is the major concern
– Likely to control for most other threats,
provided the groups are not significantly
different from one another
– See Table 9.2 for specific threats related to
each design
True Experimental Designs
Important terminology
– Random assignment
Subjects placed into groups by random
Ensures equivalency of the groups
– Random selection of subjects
Subjects chosen from population by random
Ensures generalizability to the population from
which the subjects were selected (i.e. external
validity)
True Experimental Designs
Types
– Randomized post-test only experimental control
groups
RA
RB
X O
O
– Randomized post-test only multiple treatment
groups
R A
R B
X1 O
X2 O
True Experimental Designs
Types (continued)
– Randomized pretest/post-test multiple
treatment groups
R A O X1 O
R B O X2 O
– Randomized pretest/post-test experimental
control groups
R A O X O
R B O
O
True Experimental Designs
Threats to internal validity
– Controls for selection, maturation, and
statistical regression
– Likely to control for most other threats
– See Table 9.2 for specific threats related to
each design
Evaluating Experimental
Designs
Criteria for evaluating experimental
research
– The primary purpose is to test causal
hypotheses
– There should be direct manipulation of the
independent variable
– There should be clear identification of the
specific research design
Evaluating Experimental
Designs
Criteria for evaluating experimental
research
– The design should provide maximum
control of extraneous variables
– Treatments are substantively different from
one another
– The number of subjects is dependent on or
equal to the number of treatment
replications
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