There are uncontrolled extraneous variables in the study described here. Name as many as you can. Begin by identifying the dependent and independent variables.
An investigator concluded that statistics Textbook A was better than Textbook B, after comparing two statistics classes.
One class, which met MWF at 10:00 A.M., used Textbook A and was taught by Professor X. The other class, which met for 3 hours on Wednesday evening, used Textbook B and was taught by Professor Y. At the end of the term, all students took the same comprehensive test. The mean score for the Textbook A students was higher than the mean score for the Textbook B students.