From "Consumer Expenditures in 2006", October 26, 2007.

"NOTE:  A consumer unit is defined as members of a household related by blood,
marriage, adoption, or some other legal arrangement; a single person living 
alone or sharing a household with other, but who is financially independent;
or two or more persons living together who share responsibility for at least
two out of the three major types of expenses:  food, housing, and other expenses."