There is a sharp contrast in housing standards between white and black Namibians primarily because the economic imbalance between these groups has not evened out since the end of aprtheid. A majority of the population is rural, where most dwellings are self-constructed from local materials. In a 1991 housing survey, 50% of all housing units were kraals (huts) made from pole frames and thatch or mud walls. Some kraal are plastered with cow dung. It was estimated that 58% of all households lived in these type of huts. About 40% of kraal households had seven or more members. About 33% of all housing units were detached homes, but these were only serving 30% of all households. Regional and town governments build and rent housing to migrants, but the demand has overwhelmed the supply. In the 1990s, the backlog in housing units was estimated at 45,000 units.