Each month, the U.S. Bureau of Labor Statistics boils the state of the job market down to a single number: the official unemployment rate.

It’s a quick way to assess how many people might be experiencing financial hardship, but it’s impossible to capture the experiences of everyone in the country with a single data point. Some groups will fare better than the headline numbers, while others will do worse.

That difference may help explain why some of us feel nervous about the economy even though we’ve been told repeatedly the Great Recession is over, and the official rate of unemployment is 4.9%. According to the latest local area figures, for example, unemployment in the Salt Lake City metropolitan area​was 2.8% in December, while in the Bakersfield, Calif., metro area it was 10.2%. Most economists consider a rate of 4% or 5% to be functionally full employment.

“We do have a top-side number that appears to be historically low,” said Bruce Bergman, an economist with the BLS. “But as policy makers have said, there are pockets within our economy, and various groups are not sharing the same experience.”

Critics also say the headline number doesn’t account for underemployed workers or discouraged workers who were laid off during the recession, failed to find new jobs and eventually stopped looking.

“Once you’re jobless for a year or more, you are far more likely to drop out of the labor force,” said Gregory DeFreitas, an economics professor at Hofstra University in New York. “Even though unemployment is down, a whole group has vanished from the figures.”

To help account for some of the differences, the BLS produces five alternative measurements in addition to the official unemployment rate. Some are more restrictive, and some are more expansive.

The official rate, known as U-3, falls somewhere in the middle. It classifies people as unemployed if they do not have a job, have actively looked for work in the prior four weeks and are currently available for work.

In contrast, the most restrictive measurement, known as U-1, counts people as unemployed only if they have been out of work for 15 weeks or longer based on the argument that unemployment benefits may have been exhausted, causing serious financial hardship as a direct consequence of unemployment. By this measure, overall unemployment in February was 2.1%.

At the opposite extreme is U-6, which adds discouraged workers, part-time workers who want full-time jobs, and marginally attached workers to the official U-3 figure. People with impediments such as transportation problems or child-care requirements that keep them from searching for work are considered to be marginally attached workers. By this measure, unemployment in February was 9.7%.

During the Great Recession, the official rate topped out at 10%. U-6 peaked at 17.1%, and U-1 at 5.9%.

As a point of reference, the worst conditions the country has ever experienced occurred during the Great Depression, when unemployment peaked around 25%.

The remaining BLS measurements of unemployment are U-2, enumerating job losers and people who completed temporary jobs; U-4, adding discouraged workers to the official U-3 figure; and U-5, adding discouraged and marginally attached workers to the official figure.

Some encouraging news in the current report is that the broadest measure of unemployment is down, and the official rate is about as good as it gets.

Although it may seem counterintuitive, economists say zero unemployment is undesirable.

“It gets back to the question, ‘What is full employment?’ ” Dr. DeFreitas said. “Obviously, not everybody 16 and over wants a job. So, how do you decide whether you have a fully employed economy?”

It’s a difficult question to answer because there are different kinds of unemployment, and not all are signs of a distressed economy.

Cyclical unemployment occurs when there are too few jobs to employ everyone who wants to work. This is what happens during a recession, and it’s the worst.

Structural unemployment is caused by changes that affect the labor market, such as improved technology or shifting consumer preferences. New opportunities are created, but the changes cause some people to lose jobs.

Frictional unemployment occurs as part of normal turnover in the labor market; someone who is unemployed takes time to find a desirable job or an employer waits for the best applicants.

“Full employment is tied to having no inflation or deflation pressure occurring because of the state of the economy,” said John Vahaly, an economics professor at the University of Louisville in Kentucky. “With approximately 5% unemployment, we think we avoid both problems.”

Traditionally, the worry has been that if the unemployment rate falls much lower, it could trigger price inflation because, among other pressures, a tight labor market leads to demands for higher wages. Prices will increase to keep up with rising costs.

But for now, the labor market is far from overheating.

“You can see that if you look at our Employment Cost Index, which tracks wage and salary changes,” Mr. Bergman, the BLS economist, said. “The rate of wage change is not that different from what it was in 2012, and, brace yourself, it’s around 2%.”

That may mean the economy is operating under a new paradigm. So, while 4% or 5% unemployment is traditionally considered a safe level, today, the question seems to be: How low can you go?