It was a forgettable moment. The Federal Reserve’s Chairman at the time, Janet Yellen, was asked about a key economic statistic and she just couldn’t come up with it. In September 2016, Yellen was on Capitol Hill to testify as she usually did about how things were surely getting better. Rep. Frank Guinta wanted to know what that might mean in terms of the labor force participation rate. He wanted to know what it was in the preceding month, but Yellen had to shuffle hastily through a number of papers in order to find it.

It’s easy to draw a blank when answering questions outside prepared remarks. The incident itself tells us nothing about Yellen, except that she was and is like every other public figure. Still, to have spaced on that particular number almost invites reading too much into it. Something about Freud.

The labor force participation rate is one of the key puzzles, or conundrums if you like, of the last decade plus. For Federal Reserve officials, beginning under Yellen and now continuing under her successor Jay Powell, it has been one of the few happy endings. That’s not to say it indicates full recovery, only that perhaps the bleeding after 2008 finally stopped.

Officials are placing some significant degree of emphasis on its lack of further deterioration. Janet Yellen would turn to the participation rate, in prepared remarks, of course, at her final press conference in December of last year:

Given the underlying downward trend in participation stemming largely from the aging of the U.S. population, a relatively steady participation rate is a further sign of improved conditions in the labor market. We expect that the job market will remain strong in the years ahead.

In every business cycle trough or recession, workers exit the labor market discouraged that companies are more likely to lay off employees than hire new ones. Everyone knows the economic score. When recovery happens, the process reverses because similarly both workers and prospective workers understand what businesses are doing.

One of the big mysteries, sort of, was how the labor force continued to shrink year after year. As Yellen said in December 2017, Economists have come up with certain theories to try and explain this dissonance, Baby Boomer retirement among them (drugs and nebulous skills mismatch the others).