The ratio peaked at 6.6 unemployed people per job opening in July 2009 and has been falling steadily ever since.
Remember that "unemployed" counts only people looking for a job.
Some data suggests that workers are earning more: One measure of wages and salaries rose in the first three months of the year by the most in 11 years.
USA job openings surged to a record high in March, suggesting that a recent slowdown in hiring was probably the result of employers having difficulties finding qualified workers. Yet overall hiring slipped, while quits increased.
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Yet a separate measure of average hourly pay increased just 2.6 percent in April from a year earlier, even as the unemployment rate dropped to a 17-year low of 3.9 percent. That's the last time that wages truly increased significantly, and it attracted plenty of people back into the labor market. Typically, employers should post fewer jobs as the unemployment rate falls. During the depths of the Great Recession, many workers clearly perceived quitting their jobs as way too risky.
In addition, workers may be reluctant to relocate to where the jobs are, or to accept a position with lower pay or in a different field, for example.
Some economists say they still think the spike in open jobs means that employers will have to raise pay faster in coming months. "The majority of workers who voluntarily change jobs receive a pay bump, generating stronger wage growth for job switchers". That's left more jobs unfilled.
Another good sign in the same BLS release that these numbers came from: The share of people quitting their jobs continues to trend upward. The kinds of jobs available may not match the distribution of workers' skills, locations or preferences for the kind of work they want to do.