An educational psychologist designs a demanding but fair spelling test that can be administered to grade school children. The test is found to be valid and reliable. A researcher selects the test to help evaluate a “cognitive” approach to teaching spelling. A 5th grade class is tested prior to being taught the new method at the beginning of the school year and then again after the new method is taught at the end of the year. The children show improvement on their spelling scores from the first to the second test administration, suggesting that the program was effective in helping children to learn to spell. What is a threat to the internal validity of the study?

One thing that can question the internal validity of the case above is the difficulty between the first and the second test since the psychologists never really mention it.

If, let's say that the questions on the second tests are exactly the same as the first tests or have lower difficulties compared to the first one, the better performance given by the student is not necessarily influenced by the teaching method.