Because of the impracticality of carrying out an evenly distributed survey, particularly during a war, Roberts' surveys use "cluster sampling", dividing the area into a number of randomly selected, approximately equally populated regions; a random point is chosen within each region, and a fixed number of the households closest to that point are surveyed as a "cluster". While not as accurate as an evenly distributed survey of the same number of households, this technique is more accurate than merely surveying one household for each selected point.