Here's a snippet from the WSJ's Mary Pilon on the topic:
One problem he sees with the estimates: They don't take into account deductions from income taxes or breaks in employment. Nor do they factor in debt, particularly student debt loads, which have ballooned for both public and private colleges in recent years. In addition, the income data used for the Census estimates is from 1999, when total expenses for tuition and fees at the average four-year private college were $15,518 per year. For the 2009-10 school year, that number has risen to $26,273, and it continues to increase at a rate higher than inflation.
So, there's a different problem altogether. Income estimates should certainly take major into account, and unless we track some other factors, it should be noted that this data is merely correlative.