Agroup of researchers draw a sample of 300 from a popluation of 2000. of this population, 35% have only high school degrees, 45% have 4-year college degrees, and 20% have graduate degrees. the researchers divide the population into groups based on education level and then randomly select 100 individuals with only high school degrees, 100 individuals with 4-year degrees, and 100 individuals with graduate degrees. what type of sample is this?