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. Sociologists Dennis Gilbert, William Thompson and Joseph Hickey estimate the upper middle class to constitute roughly 15% of the population. Using the 15% figure one may conclude that the American upper middle class consists, strictly in an income sense, of professionals with personal incomes in excess of $62,500 ($83,000 in 2020 dollars), who commonly reside in households with six-figure incomes. The difference between personal and household income can be explained by considering that 76% of households with incomes exceeding $90,000 (the top 20%, $140,000 to cross this threshold in 2020 dollars) had two or more income earners. In 2020, the threshold for entering the top 15% of American household incomes is $166,000 Note that the above income thresholds may vary greatly based on region due to significant differences in average income based on region and urban, suburban, or rural development. In more expensive suburbs, the threshold for the top 15% of income earners may be much higher. For example, in 2006 the ten highest income counties had median household incomes of $85,000 compared to a national average of about $50,000. The top 15% of all US income earners nationally tend to be more concentrated in these richer suburban counties where the cost of living is also higher. If middle-class households earning between the 50th percentile ($46,000) and the 85th percentile ($62,500) tend to live in lower cost of living areas, then their difference in real income may be smaller than what the differences in nominal income suggest. Values Upper-middle-class people tend to highly value tertiary education for themselves and their children, favoring the pursuit of undergraduate and postgraduate degrees. Political ideology is not found to be correlated with social class; however, a statistical relationship is seen between the level of one's educational attainment and one's likelihood of subscribing to a particular political ideology. In terms of income, liberals tend