While many Americans cite income as the prime determinant of class, occupational status, educational attainment, and value systems are equally important variables. Income is in part determined by the scarcity of certain skill sets. An occupation that requires a scarce skill set which is attained through higher educational degree, and which involves higher autonomy, responsibility and influence, will usually offer higher economic compensation. Qualifying for such higher income often requires that individuals obtain the necessary skills (e.g., by attending law, medical, or postgraduate school) and demonstrate the necessary competencies. There are also differences between household and individual income. In 2005, 42% of US households (76% among the top quintile) had two or more income earners; as a result, 18% of households but only 5% of individuals had six-figure incomes. To illustrate, two nurses each making $55,000 per year can out-earn, in a household sense, a single attorney who makes a median of $95,000 annually.