Note to EPSY 341 Students: An understanding of the difference between
population and ecological validity is sufficient. Mastery of the sub categories for each
is not necessary for this course.
--to whom can the results of the study be applied--
There are two types of study validity: internal (more applicable with
experimental research) and external. This section covers external validity.
External validity involves the extent to which the results of a
study can be generalized (applied) beyond the sample. In other words, can you apply what
you found in your study to other people (population validity) or settings (ecological
validity). A study of fifth graders in a rural school that found one method of
teaching spelling was superior to another may not be applicable with third graders
(population) in an urban school (ecological).
Threats to External Validity
Population Validity the extent to which the results of a study can be generalized from
the specific sample that was studied to a larger group of subjects
1. the extent to which one can generalize from the study
sample to a defined population--
If the sample is drawn from an accessible population,
rather than the target population, generalizing the research results from the accessible
population to the target population is risky.
2. the extent to which personological variables interact with
If the study is an experiment, it may be possible that
different results might be found with students at different grades (a personological
Ecological Validity the extent to which the results of an experiment can be generalized
from the set of environmental conditions created by the researcher to other environmental
conditions (settings and conditions).
- Explicit description of the experimental treatment
(not sufficiently described for others to replicate)
If the researcher fails to adequately describe how he or she
conducted a study, it is difficult to determine whether the results are applicable to
- Multiple-treatment interference (catalyst
If a researcher were to apply several treatments, it is
difficult to determine how well each of the treatments would work individually. It might
be that only the combination of the treatments is effective.
- Hawthorne effect (attention causes
Subjects perform differently because they know they are being
studied. "...External validity of the experiment is jeopardized because the findings
might not generalize to a situation in which researchers or others who were involved in
the research are not present" (Gall, Borg, & Gall, 1996, p. 475)
- Novelty and disruption effect (anything
different makes a difference)
A treatment may work because it is novel and the subjects
respond to the uniqueness, rather than the actual treatment. The opposite may also occur,
the treatment may not work because it is unique, but given time for the subjects to adjust
to it, it might have worked.
- Experimenter effect (it only works with
The treatment might have worked because of the person
implementing it. Given a different person, the treatment might not work at all.
- Pretest sensitization (pretest sets the
A treatment might only work if a pretest is given. Because
they have taken a pretest, the subjects may be more sensitive to the treatment. Had they
not taken a pretest, the treatment would not have worked.
- Posttest sensitization (posttest helps
treatment "fall into place")
The posttest can become a learning experience. "For example, the posttest might cause
certain ideas presented during the treatment to 'fall into place' " (p. 477). If the
subjects had not taken a posttest, the treatment would not have worked.
- Interaction of history and treatment effect
(...to everything there is a time...)
Not only should researchers be cautious about generalizing to other population, caution
should be taken to generalize to a different time period. As time passes, the conditions
under which treatments work change.
- Measurement of the dependent variable
(maybe only works with M/C tests)
A treatment may only be evident with certain types of measurements. A teaching method may
produce superior results when its effectiveness is tested with an essay test, but show no
differences when the effectiveness is measured with a multiple choice test.
- Interaction of time of measurement and treatment
effect (it takes a while for the treatment to kick in)
It may be that the treatment effect does not occur until several weeks after the end of
the treatment. In this situation, a posttest at the end of the treatment would show no
impact, but a posttest a month later might show an impact.
Bracht, G. H., & Glass, G. V. (1968). The external validity of
experiments. American Education Research Journal, 5, 437-474.
Gall, M. D., Borg, W. R., & Gall, J. P. (1996). Educational
research: An introduction. White Plains, NY: Longman.
Del Siegle, Ph.D.
Neag School of Education - University of Connecticut