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 fourth most common?