Further investigation may compare regional convergence of income to regional convergence of salaries and wages, as the IRS tables also reveal income composition by state. Section A.4 of the appendices suggests a beginning of analysis towards that goal.
On the one hand, average income and income of top 1 percent both record a sharp decrease in dispersion across states over time (except after the mid 1980s). On the other hand, the incomes of the top 90-95 and 95-99 percent feature an increase in dispersion across states over time. The analysis of such an opposition remains to be done.
On Chapter 6, the assumption of the Pareto distribution applied to the entire income distribution is questionable. More assumptions on the functional form of the Lorenz curve need to be tested, inasmuch as the hypothesis influences the results and affects their economic interpretation to a certain extent.
Finally, the IRS tables record tax liabilities data consistently from 1913 on. Several aspects of the tax policies implemented over the past century could be explained with those data that remain to be computed.