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.