To divert attention from the disastrous rollout of his health reform, President Obama has decided to change the national conversation to discuss increasing inequality. This phenomenon is not new--the trend started about four decades ago--but it is real and important. In case you are a new reader of this blog, you can find my personal views on the matter in this paper This national conversation has generated renewed attention to the highly influential Piketty-Saez data. It is worth pointing out, therefore, some limitations of these data, which have been stressed by Cornell economist Richard Burkhauser : The data are on tax units rather than households, they do not include many government transfer payments, they are pre-tax rather than post-tax, they do not adjust for changes in household size, and they do not include nontaxable compensation such as employer-provided health insurance.Does this matter? Yes! Here are some numbers from the Burkhauser paper:1. From 1979 to 2007, median real income as measured by pre-tax, pre-transfer cash income of tax units rose by only 3.2 percent. That is a paltry amount for such a long period. You might conclude that middle class incomes have been stagnant. But wait.2. Households are more important than tax units. Two married people are one tax unit, whereas a couple shacked up are two tax units. We would not want to treat the movement from marriage to shacking up as a drop in income. If we look at households rather than tax units, that meager 3.2 percent rises to a bit more respectable 12.5 percent.3. Now consider government transfer payments. If we add those in, that 12.5 percent number becomes an even better 15.2 percent.4. What about taxes? The middle class received some tax cuts during that period. Factoring taxes in, the 15.2 percent figure rises to 20.2 percent.5. But not all households are the same size, and the size of households has fallen over time. Adjusting for household size increases that 20.2 percent to 29.3 percent.6. There is still one thing left: employer-provided health insurance, an important fringe benefit that has grown in importance. Adding an estimate of that into income raises the 29.3 percent figure to 36.7 percent.So, during this period, has the middle class experienced stagnant real income (a mere 3.2 percent increase) or significant gains (a 36.7 percent increase)? It depends on which measure of income you look at. It seems clear to me that the latter measure is more relevant, but the former measure of income often gets more attention than it deserves.Take this as a cautionary tale. When people talk about changes in income over time, make sure you know what measure of income they are citing.