Type of sampling that uses the differences that already exist in a population as the basis for selecting a sample; knowing the percentage of the population that falls into a particular category, the researcher then randomly selects a number of persons to be studied from each category in the same proportion as exists in the population
A form of polygamy in which there is a marriage between two or more men and two or more women
Piaget's stage of development in which the child develops the capacity for thinking in highly abstract terms of metaphors and hypotheses