In phase I testing of a new drug designed to increase the red blood cell count, a researcher obtains envelopes with the names and addresses of all treated subjects. She wants to increase the dosage in a sub-sample of 32 subjects, so she thoroughly mixes all of the envelopes in a bin, then pulls 32 of those envelopes to identify the subjects to be given the increased dosage. What type of sampling is used? O Cluster sample O Convenience sample Stratified sample O Systematic sample O Simple random sample