Dr. Espinoza wanted to know if exposure to the risks of bullying helped reduce bullying at her school. To explore this question she randomly select 200 students in her school and randomly enrolled half the students in an 8-week anti-bullying course; the other half kept their regular course schedule. At the end of the 8 weeks, she had all 200 students complete a survey on the number of anti bullying classes they attended over 8-weeks and their attitudes toward bullying. Results showed that the more anti-bullying classes a student attended, the less they endorsed bullying (r=-.71). What sampling technique did Dr. Espinoza use