Dr. Lawrence is the director of Counseling Services at her university. She is planning to conduct a survey of 100 students at the university to see how aware they are of the counseling services offered at the university. She wants the proportion of men and women in her sample to reflect the proportion in the university as a whole (55% women and 45% men). Dr. Lawrence plans to stand in the Student Union and ask people to participate until she has given the survey to 55 women and 45 men. Which type of sampling method is Dr. Lawrence going to use?