In 2000 there were 512,253 households out of which 31.70% had children under the age of 18 living with them, 48.70% were married couples living together, 11.80% had a female householder with no husband present, and 33.70% were non-families. 24.50% of all households were made up of individuals and 6.70% had someone living alone who was 65 years of age or older. The average household size was 2.65 and the average family size was 3.17.. which household was least common?