It's also common knowledge that different careers earn different levels of income. (loans.org)
This held true even when controlling for factors like different levels of income, education, race and family status. (badcredit.org)
Progressivity could be embodied in the rate structure (with rates rising with income, as they do now), a basic exemption amount, and the choice of tax base (income, consumption, or another measure), rather than through specific provisions that treat different levels of income and consumption differently. (taxpolicycenter.org)