A researcher is conducting an experimental study on the effect of ibuprofen on heart attack risk. In order to assign who will receive ibuprofen and who will receive a placebo, the researcher first divides the study into three age groups (18-29, 3049, and 50 or older). Then the researcher randomly assigns individuals within these three age groups to either receive ibuprofen or the placebo. This is an example of a _______ sample