Cognisonance: I'm not trained in even basic statistics, so I can't comment on your work, although it may be that larger samples would be more meaningful. Nevertheless, I think the real import is that these trends seem to corroborate what many of us have suspected or observed in recent years.
Congs. seem to be filled with women and/or elderly people. And we can look at the increase of Spanish-lang. district conventions as an indicator of the growth in that demographic. Women, elderly and minorities tend to make less money than other groups. Moreover, trends in employment point to fewer and lower-paying jobs available for people with only high school educations. IOW, these Pew statistics fit with the overall picture.
I think the GB has reason to be very alarmed--we will likely see even more fervent and frequent pleas for money but as these stats suggest, they are going after the ones who have least to give (and are often the most willing and vulnerable).