The American economy grew dramatically in the post-war period, expanding at a rate of 3.5% per year between 1945 and 1970. During this period of prosperity, many incomes doubled in a generation, described by economist Frank Levy as "upward mobility on a rocket ship." The substantial increase in average family income within a generation resulted in millions of office and factory workers being lifted into a growing middle class, enabling them to sustain a standard of living once considered to be reserved for the wealthy. As noted by Deone Zell, assembly-line work paid well, while unionized factory jobs served as "stepping-stones to the middle class." By the end of the 1950s, 87% of all American families owned at least one T.V., 75% owned cars, and 60% owned their homes. By 1960, blue-collar workers had become the biggest buyers of many luxury goods and services. In addition, by the early-1970s, post-World War II American consumers enjoyed higher levels of disposable income than those in any other country.