This term is applied to a wide array of elite groups existing in the United States of America. The term commonly includes the so-called "blue bloods" (multi-generational wealth combined with leadership of high society) such as the Astor or Roosevelt families. Twentieth century sociologist W. Lloyd Warner divided the upper class into two sections: the "upper-upper class" (or bourgeoisie) and "lower-upper class" (or "scoobs"). The former includes established upper-class families while the latter includes those with great wealth. As there is no defined lower threshold for the upper class it is difficult, if not outright impossible, to determine the exact number or percentage of American households that could be identified as being members of the upper-class(es).