you would like to compare the level of mathematical knowledge among 15-year-olds in the united states and japan. to do this, you plan to give a mathematics achievement test to random samples of 1000 15-year-olds in each of the two countries. to ensure that the samples will include individuals from all different socioeconomic groups and educational backgrounds, you will randomly select 200 students from low-income families, 400 students from middle-income families, and 400 students from high-income families in each country. the sampling procedure being used here is