Anyone who follows the media has probably heard many times that the rich are getting richer, the poor are getting poorer, and incomes of the population in general are stagnating. Moreover, those who say such things can produce many statistics, including data from the Census Bureau, which seem to indicate that.

On the other hand, income tax data recently released by the Internal Revenue Service seem to show the exact opposite: People in the bottom fifth of income-tax filers in 1996 had their incomes increase by 91 percent by 2005.

The top one percent -- "the rich" who are supposed to be monopolizing the money, according to the left -- saw their incomes decline by a whopping 26 percent.

Meanwhile, the average taxpayers' real income increased by 24 percent between 1996 and 2005.

How can all this be? How can official statistics from different agencies of the same government -- the Census Bureau and the IRS -- lead to such radically different conclusions?

There are wild cards in such data that need to be kept in mind when you hear income statistics thrown around -- especially when they are thrown around by people who are trying to prove something for political purposes.

One of these wild cards is that most Americans do not stay in the same income brackets throughout their lives. Millions of people move from one bracket to another in just a few years.

What that means statistically is that comparing the top income bracket with the bottom income bracket over a period of years tells you nothing about what is happening to the actual flesh-and-blood human beings who are moving between brackets during those years.

That is why the IRS data, which are for people 25 years old and older, and which follow the same individuals over time, find those in the bottom 20 percent of income-tax filers almost doubling their income in a decade. That is why they are no longer in the same bracket.

That is also why the share of income going to the bottom 20 percent bracket can be going down, as the Census Bureau data show, while the income going to the people who began the decade in that bracket is going up by large amounts.

Unfortunately, most income statistics, including those from the Census Bureau, do not follow individuals over time. The Internal Revenue Service does that and so does a study at the University of Michigan, but they are the exceptions rather than the rule. Continued...