Question

An educational psychologist is asked to conduct a study to determine if class size has any effect on academic achievement. The psychologist identifies a large fifth-grade classroom in one inner-city school and a small fifth-grade classroom in another inner-city school. Principals from each school agree to participate in the study and select the teachers and classes. At the end of the school year, the researcher obtains achievement test scores from students in both schools and compares the mean achievements of the two groups.

a. Based solely on the preceding description, what threats to internal or external validity exist in this study? Explain
b. Can you think of an alternative method that would strengthen the internal and external validity of this study?

Answer 1

(a)

Threats to internal validity: The teaching capability of the teachers of both schools and the inherent intelligence of each student will be different Thus, the effects observed in the study (here.,achievement test scores from students in both schools) are due to not only the manipulation of the independent variable (here: class size) but also due to other factors (here: teaching capability of the teachers of both schools and the inherent intelligence of each student).

Threat to external validity: The result of the study can be generalized subject to ecological validity, population validity and historical validity.

(b)

Alternative method that would strengthen the internal and external validity of this study:

Instead of randomly choosing the schools and teachers, select teachers of equal competence. and students of equal intelligence.