Final answer:
The researcher's encounter with having to switch classes due to field trips illustrates issues of sample selection, data validity, and potential sampling bias, affecting the internal validity of the study on the impact of a study skills course.
Step-by-step explanation:
You examined the IQ scores of students at several high schools and matched classes from each of grades 9 to 12 from two high schools to analyze the impact of a study skills course. The issue you've encountered with having to switch to different classes due to the original ones being on field trips illustrates problems of sample selection and data validity. Since you did not get to work with the classes you originally selected, the replacement classes may not be comparable in all respects, which could affect the internal validity of your study. Specifically, this can introduce sampling bias because the replacement classes may have different characteristics that could influence the outcome of the study, compromising the ability to attribute differences in performance to the study skills course.