Economic class is highly correlated with social, religious, ethnic, tribal class. If there are more social, religious, ethnic, tribal classes in a single countr

Economic class is highly correlated with social, religious, ethnic, tribal class. If there are more social, religious, ethnic, tribal classes in a single country (i.e. the country is diverse) then in another, then you do need to sample more of the groups to get sample means free from systematic bias. Simple example: image you are asked to compute the mean diameter of all balls in each of two bags. You open the first bag, and it is all blue balls, and they all look very close in radius. So you measure a few and call it a day. You open the second bag, and there are balls of about 50 different colors, and it seems like all the colors have different radius. So, then to get a good result, you need to measure almost the diameter of most of the colors.