General Discussion

The median income decreased during the recession, so the standard for qualifying as "middle class" also decreased. A middle class individual making the median income in 2010 was worse off than an individual making the median income in 2007. Similarly, when everyone is doing well, it also skews the numbers. If the bulk of the population moves to a higher income the median increases, and people who were formerly middle class might drop to the lower income class, even though their circumstances haven't changed.

But the federal government doesn't use median income statistics to determine poverty levels, nor do most states (as far as I know); they do look at actual costs of living versus income to establish a variety of poverty thresholds for a variety of circumstances.

One good use of the statistical chart for upper, lower and middle classes is to see how income inequality changes over time. Regardless of the position of the middle, the measure of income inequality says a great deal about the amount of opportunity in a society, and how it is composed. From the chart I got the numbers from, the simple statement could be made: while all three classes increased in size over time, the upper and lower classes increased at a greater rate than the middle class. That indicates an increase in income inequality, which changes the character of society in a bad direction.

(on edit: link http://www.pewresearch.org/fact-tank/2015/02/04/americas-middle-holds-its-ground-after-the-great-recession/ , as I didn't post it as the source of data originally)