人的一生要从事多少种工作?

Do Americans really go through careers like they do cars or refrigerators?

As workers take in the latest round of monthly unemployment data over Labor Day weekend, Americans are focused on volatility in the job market. Much of what they hear points to growing job instability and increased autonomy of workers. Among the most-repeated claims is that the average U.S. worker will have many careers -- seven is the most widely cited number -- in his or her lifetime.

Jobs researchers say the basis of the number is a mystery. 'Seven careers per person sounds utterly implausible to me,' says Ann Stevens, professor and chair of the economics department at the University of California, Davis.

Yet the estimate has had extraordinary staying power. One reason is that no one knows for sure the true average number of careers. The U.S. Bureau of Labor Statistics, the Labor Department's data arm, doesn't track lifetime careers. Even so, the figure is erroneously attributed to BLS so often that the agency includes a corrective memo on its website, explaining that 'no consensus has emerged on what constitutes a career change.'

What researchers do know is that job changes are common early in a person's working years: Three in four workers age 16 to 19, and half between 20 and 24, have been with their current employers for under a year, the BLS says.

But early, frequent switches, which can include jumps by students between summer jobs, aren't what many people would consider career changes in the same way as a midlife switch after a decade or two in the same job. The latter type can entail major costs in retraining and pay cuts -- plus, in the current job climate, the risk of not finding employment. It is difficult to imagine, researchers say, that the typical worker undertakes a major switch seven times.

Surveys of workers could be easily skewed by a small number of zealous career changers. Adding to the confusion, economists say, is that workers sometimes take on enough new responsibilities to meet a technical definition of a career change without leaving their general field.

The BLS offers on its website the example of an economist who is promoted to an administrative position, changing her job function even if her title remains economist.

'The problem is career change is tricky to define,' says Solomon Polachek, a professor of economics and political science at Binghamton University in New York, who nonetheless calls the seven-career figure 'a considerable overestimate.'

And without hard data, anecdotal reports that point to an expanding career-change mentality in the U.S. have taken on a life of their own. The notion of continual career switches is repeated in particular by career-management experts, whose jobs involve spending a lot of time with occupation switchers. 'Based on my experience, I believe the typical person has six to seven careers, and the number is growing,' says Jeff Neil, a New York City career counselor, in an email.

He describes one 30-year-old client, currently working as a manager of a doctor's office, who is exploring a new professional path. Previously, she worked in real-estate sales, at a talent agency, a sports-car dealership and as a sales representative at top-end health clubs. Mr. Neil adds that while multiple shifts are more the norm than the exception, he couldn't say for certain without formally researching the issue.

While data on career changes are scarce, economists and statisticians have examined how often Americans shift jobs. The U.S. Census Bureau asks some respondents to its Current Population Survey who are employed how long they have been 'working continuously' for their current employer.

These surveys have been used by researchers at the Bureau of Labor Statistics and elsewhere to count the total number of jobs in a lifetime. Their findings suggest that job stability hasn't changed all that much in the U.S. since the late 1990s. For example, the typical American worker's tenure with his or her current employer was 3.8 years in 1996, 3.5 years in 2000 and 4.1 years in 2008, the latest available data.

BLS economist Chuck Pierret has been conducting a study to better assess U.S. workers' job stability over time, interviewing 10,000 individuals, first surveyed in 1979, when group members were between 14 and 22 years old. So far, members of the group have held 10.8 jobs, on average, between ages 18 and 42, using the latest data available.

Dr. Pierret points out that these workers' experience might not apply to entrants to today's job market. The bureau is just starting to track job changes for people born between 1980 and 1984. But in yet another example of the difficulty of measuring career stability, the recent recession may have skewed things so much that long-term trends will be masked.

'Enough of their working lives have been affected by the downturn,' Dr. Pierret says of the 20-somethings in the study, 'that it may be not so comparable.'