Assuming the set is homogeneous! Which in this case, is not. A sample extracted from a more or less financially stable region of the country will have drastical

Assuming the set is homogeneous! Which in this case, is not. A sample extracted from a more or less financially stable region of the country will have drastically different output than a sample extracted from a poorer area. Your assumption about 'the soup well mixed' is less probable to be true, given the lack of any evidence supporting the claim that this study is either meaningful or properly executed.