Many Americans believe in a social class system that has three different groups or classes: the American rich, the American middle class, and the American poor. More complex models propose as many as a dozen class levels, including levels such as high upper class, upper class, upper middle class, middle class, lower middle class, lower class and lower lower middle class. while others disagree with the American construct of social class completely. Most definitions of a class structure group its members according to wealth, income, education, type of occupation, and membership within a hierarchy, specific subculture, or social network. Most concepts of American social class do not focus on race or ethnicity as a characteristic within the stratification system, although these factors are closely related.