Direction: The questions in this section are based on the reasoning contained in brief statements or passages. For some questions, more than one of the choices could conceivably answer the question. However, you are to choose the best answer; that is, the response that most accurately and completely answers the question. You should not make assumptions that are by common sense standards implausible, superfluous, or incompatible with the passage.
An author writing a book about birth order advertised in his local paper for people who were first born. Fifty people consented to be interviewed and assessed for certain personality traits. As the writer suspected, the interview results and personality assessments showed that first-borns were more goal-oriented and serious-minded than random samples of the general public. These findings support the conclusion that people are affected by their birth order.
Which one of the following selections, if true, points out the most critical weakness in the method used by the author to investigate birth order characteristics?