A researcher was interested in the exercise habits of the residents of the village of Pine Heights. She interviewed every 10 th person who left the post office until she had interviewed 100 people. Each of the people interviewed was asked to rate their exercise level according to the following scheme "(1) very little, (2) some, (3) moderate, (4) a lot, (5) extensive". The researcher reported that, on average, the residents of Pine Heights exercise moderately. What is the biggest flaw in the study? Participation bias Selection bias Measurement of the variable is not well defined Confounding variables The source of the study.