You can compute the median wage for Blacks, Catholics, Left-handed Lesbian Windshield Installers, or any other identifiable group, for which the pertinent records are kept.
The bottom 80% of the population is just another group.
The median wage of just about any group will change over time. Unfortunately, for the bottom 80% of the population, it went down 4% from 1999 to 2003, in real, inflation-adjusted terms. This while the total economy is growing nicely.
This should not be a Democrat/Republican, liberal/conservative, scream and yell, pointing fingers blame game.
There is something, or some effect, in the economy, that is screwing over the middle-class, and, whatever it is, we need to identify it, and clip it in the bud, before our middle class gets a nice taste of Mexican peonage. (Shoot, maybe that IS the idea.)
What is the point of disregarding 20% of the available data, if not to skew the calculation?