Since 1980, the pre-tax median income of the two-earner family has leaped to $46,848 from $29,627, a sizable increase until you try to spend it.

Don't try. A lot of people who attempt doing so end up in bankruptcy court.While the figures cited are real as mathematics can make them, they are, for the practical purposes of every day life, as reliable as a swindler's spiel. There is a wide gap between perception and reality.

In a sense, much income today is pre-spent on taxes at all levels and on Social Security levies. Moreover, what remains is reduced sharply by inflation, which erodes spending power as relentlessly as termites turn wood to dust.

In real terms - after subtracting for inflation and higher direct federal taxes, including Social Security - the median income for a two-earner household with two children has risen a bit more than $3,700 so far in the 1980s.

That figure, from the Tax Foundation, a non-profit organization that claims to have no ax to grind except that which is used to hack away at inefficiency, is but a fraction of the $14,000-a-year increase depicted by the raw figure.

The disparity between appearance and reality might help explain the tendency toward low savings and extensive use of credit that has identified the 1980s as distinct from other recent decades.

In explanation, it might be argued that a lot of folks tend to think of themselves as financially better off than they are, and then attempt to live up to that image. If it takes credit to do so, then so be it.

Some, in fact, may have justification for living beyond their job incomes, having made wise investments in securities or real estate that provide them with dividends, borrowing power or saleable assets.

But, based strictly on income earned from jobs, the financial story of American families is not as pretty as it might at first seem.

The Tax Foundation study says that in 1980 the median two-earner family with two children, and filing a joint return, paid $4,050 in federal income taxes and $1,816 in Social Security taxes, for a total bite of $5,866.

In 1988, the estimate for the $46,848 median income is $5,623 for federal income taxes and $3,518 for Social Security taxes. The total of $9,141 supposedly will leave $37,707 to be used by the family.

The results, however, are illusory, since they are stated in current dollars.

That is, the 1980 figures were in 1980 dollars, and the 1988 numbers were measured by the 1988 dollar. The calibrations on the measuring stick were different - like saying a foot was 12 inches one time and six inches another.

What changes the calibrations on the measuring stick, of course, is inflation. And while inflation during the 1980s has not been nearly as bad as in the double-digit 1970s, it does add up - and it compounds, too.

The only way to make a sensible comparison is to use the same measuring stick, and the Tax Foundation chose the 1988 dollar. By that measurement, the 1988 after-tax income remains $37,707; but the 1980 income becomes $33,393.

It could be stated differently. The value of the dollar in any year could have been used to make the comparisons. The important thing is that the same measuring device, the same dollar, be used for both periods.

Even then the figures obtained might not reflect reality. The foundation study, for example, included only direct federal taxes. It omitted state and local income taxes, to say nothing of real estate and sales taxes.