A researcher is conducting an experiment to investigate the impact of a new teaching method on student performance. The study involves two groups: one exposed to the new teaching method and another following the traditional method. The researcher lets students self-select which method they want to be a part of. What critical mistake did the researcher make in conducting the experiment that undermined its validity? Group of answer choices No random assignment of participants to experimental groups Lack of a clear hypothesis Inadequate monitoring of participant engagement Inadequate sample size