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 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 children to learn to spell. what is a threat to the internal validity of the study?