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A validity case study

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An educational psychologist is asked to conduct a study to determine if class size has an 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.

Based solely on the preceding description, this job lists threats to internal or external validity exist in this study and any alternative method that would strengthen the internal and external validity of this study.

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a) Based solely on the preceding description, what threats to internal or external validity exist in this study? Explain please.
You can think of internal validity as being any issue that would interfere with you actually measuring what you think you're measuring. So a study with low internal validity would appear to indicate a difference due to the variable of interest, when in fact that difference is due to some other factor or variable.

In your example, the researcher is interested in whether class size influences academic achievement. In order to draw conclusions about this relationship, and to feel confident that any differences in academic achievement are due to differences in class size, the researcher should ...

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