To test the effect of a new dietary plan (reducing sugar intake) to improve short term memory, a researcher sets up the following experiment: 100 volunteers, 50 high school students and 50 senior citizens. They are shown ten words on a piece of paper and given 30 seconds to memorize the words. The participants wait 5 minutes, and then are asked to write as many words as they can remember. The researcher uses the high school students as the experimental group, having them utilize the new dietary plan for one month before administering the quiz again. The senior citizens are not given a new dietary plan and are tested again after one month. The results of this experiment are likely to be invalid because...