I'm a little confused. First of all, if you are in a two-income household do you use the mean of the two incomes to determine which quintile you're in?
Also, doesn't geography matter? I mean, I live quite comfortably in Tucson, AZ and was able to buy a house with my modest income. I have friends in NY and CA, however, who make more than I do but their cost of living is much higher. My brother in CT paid at least 3 times the amount that I did for his house and the houses are similar in size. So someone in Middleofnowheresville in middle America could be "middle class" making the same amount as a poor person in San Francisco, no?