Why has no one thought to do this before? Every month, the Current Population Survey goes out to a nationally representative sample of more than 50,000 interviewed households and their members. And in one of the questions, those households — or at least the households who didn’t answer the same question the previous month — are asked how much money they made, in total, over the past 12 months. That question has now been asked in 138 successive months, since January 2000. Which means that with a bit of clever analysis, it’s possible to put together an apples-to-apples comparison of what has happened to household income every month.

And when you do that, the results are very scary indeed.

The red line, here, is median real household income, as gleaned from the CPS, indexed to January 2000=100. It’s now at 89.4, which means that real incomes are more than 10% lower today than they were over a decade ago.

More striking still is the huge erosion in incomes over the course of the supposed “recovery” — the most recent two years, since the Great Recession ended. From January 2000 through the end of the recession, household incomes fluctuated, but basically stayed in a band within 2 percentage points either side of the 98 level. Once it had fallen to 96 when the recession ended, it would have been reasonable to assume some mean reversion at that point — that with the recovery it would fight its way back up towards 98 or even 100.

Instead, it fell off a cliff, and is now below 90.

In dollar terms, median household income is now $49,909, down $3,609 — or 6.7% — in the two years since the recession ended. It was as high as $55,309 in December 2007, when the recession began.

Some of this decline has been hard to see because nominal incomes have been holding very steady: before taking inflation into account, median household income was $51,465 in December 2007, and $51,140 in June 2009. But even then, over the past two years, nominal incomes have shrunk significantly to the current level of $49,909.

All of these numbers come from Gordon Green and John Coder, economists who both worked at the Census Bureau for more than 25 years. They’ve now set up a private company, Sentier Research, to collate these household income figures every month; the full report costs a reasonable $20.

Why is this work being outsourced to private-sector economists, rather than being done by the Bureau of Labor Statistics and published officially? I’m having dinner with a government statistics wonk on Wednesday, and will be sure to ask him.

But in the absence of any good reason to discount the reliability of these numbers, it’s definitely worth taking them seriously, and asking why incomes have eroded so quickly and dramatically over the past two years. We’ve known for years that America has a huge unemployment problem. But I had no idea that the plight of the employed was this bad.