Dr. Schwartz is assessing a healthy-eating program that requires participants to eat a large amount of fruits and vegetables. In the first phase, Dr. Schwartz observes the participants while they eat their normal diet. In the next phase, Dr. Schwartz adds fruits, then adds vegetables in the following phase. Dr. Schwartz makes the appropriate levels of observations and then returns the participants to the baseline phase and then repeats the process of adding first fruits and then adding vegetables as well. Which research design is she using?