Economists do not agree on how to measure income:

The biggest point of contention between the two camps revolves around “unreported income,” more commonly known as tax evasion. Tax returns are the best data source available for studying income distributions, but they’re incomplete—most obviously because people don’t report all of the income that they’re supposed to. This information gap requires inequality researchers to make some educated guesses about how unreported income is distributed, which is to say, about who is evading the most taxes. Piketty, Saez, and Zucman assume that it’s the people who already report a lot of income: Think of the well-paid corporate executive who also stashes millions of dollars in an offshore account. Auten and Splinter, by contrast, assume that those who evade the most taxes are people who report little or no income: Think plumbers or housekeepers who get paid in cash. They believe, in other words, that members of the 99 percent are a lot richer than they look…
To take the true measure of inequality, economists need a way to account for all the income and expenses that don’t show up on people’s tax returns. The method that Piketty, Saez, and Zucman pioneered, and that Auten and Splinter follow, was to take the gross domestic product—a measure of all of the spending in the national economy every year—and figure out who exactly is receiving how much of it. (Technically, they use something called gross national income, which is a close cousin of GDP.) The benefit of this approach is that nothing gets left out. The drawback is that, well, nothing gets left out. GDP measures the total production of an entire economy, so it includes all sorts of expenditures that don’t seem like income at all.
Much of the difference between the authors’ estimates of inequality hinges on how they treat government spending on things that benefit the public at large, such as education, infrastructure, and national defense. Because this spending is part of gross national income, it must be allocated to someone in order for the math to work out. Piketty, Saez, and Zucman take the view that this stuff really shouldn’t be considered income, so they allocate it in a way that doesn’t change the overall distribution. Auten and Splinter, however, argue that at least some of this money should count as income. Citing research indicating that education spending tends to disproportionately benefit lower- and middle-income kids, they decide to allocate the money in a way that increases the bottom 99 percent’s share of income—by a lot. Austin Clemens, a senior fellow at the Washington Center for Equitable Growth, calculates that in Auten and Splinter’s data set, a full 20 percent of income for those in the bottom half of the distribution “comes in the form of tanks, roads, and chalkboards.”…
The deeper you get into how GDP is actually calculated and allocated, the more you feel as though you’ve fallen through a wormhole into an alternate dimension. Let’s say you own a house. Government statisticians imagine that you are renting out that house to yourself, calculate how much money you would reasonably be charging, and then count that as a form of income that you are, in essence, paying yourself. This “imputed rent” accounts for about 9 percent of all GDP, or more than $2 trillion. Or suppose you have a checking account at a major bank. Statisticians will calculate the difference between what the bank pays you in interest on that account (usually close to nothing) and what you could have earned by investing that same money in safe government bonds. That difference is then considered the “full value” of the benefits you are receiving from the bank—above and beyond what it actually charges you for its services—and is therefore considered additional income for you, the depositor. All of these choices have some theoretical justification, but they have very little to do with how normal people think about their financial situation.
These are common issues working with all sorts of variables that matter in life: trying to collect good data, operationalization, missing data, judgment calls, and then difficulty in interpreting the results. In this case, it affects public perceptions of income inequality and big questions about the state of society.
Is this just an arcane academic debate? Since academics tend to want their work to matter for society and policy, this particular discussion matters a lot. Every day, economic news is reported. People have their own experiences. Humans like to compare their own experiences to those of others now and in the past. People search for certainty and patterns. The question of inequality is a recurrent one for numerous reasons and having good data and interpretations of that data matters for perceptions and actions.
The way that academics tend to deal with this is to continue to measure and interpret. Others will see this debate and find new ways to conceptualize the variable and collect data. New studies will come out. Scholars of this area will read, discuss, and write about this issue. There will be disagreement. Conditions in the world will change. And hopefully academics will get better at measuring and interpreting the concept of income.
