Dr. Padilla wants to measure attitudes related to scientific productivity among attendees at a national conference on higher education. To narrow the sampling frame, Dr. Padilla divides the participant list into regions of the country and randomly selects two states from each region. She then divides each state list into public and private universities and randomly selects two universities from each state. Finally, Dr. Padilla selects 50 participants randomly from this narrowed list. This type of sampling would be considered