WASHINGTON: More than 30 per cent of American adults hold bachelor's degrees, a first in the history of the United States, according to the latest census figures.

Asian Americans remain the nation's best-educated racial group, with 50.3 per cent having such degrees and 19.5 per cent holding graduate degreesBut the figures also showed that in terms of future earnings, education level matters less these days than in previous generations and the field of study matters more.

College attainment has crept higher, slowly but steadily, in the US. In 1947, just 5 per cent of Americans aged 25 and older held degrees from four-year colleges. As recently as 1998, fewer than one-quarter of the adult population held college degrees.

'We believe this is a notable milestone,' said Mr Kurt Bauman, chief of the US Census Bureau's education and social stratification branch, during a conference on Thursday to announce the data.

The Washington, DC region remains the best-educated US metropolis. As of 2010, 46.8 per cent of adults in the area held at least a bachelor's degree, the highest rate among the 50 largest metro areas. California's Silicon Valley ranked second, with 45.3 per cent college attainment.

In 2009, President Barack Obama set a national goal of reclaiming the world lead in college attainment, which the US once held. But instead of gaining ground, the country has fallen in global rank, slipping from 12th to 16th in the share of people aged 25 to 34 holding college degrees, said the Organisation for Economic Cooperation and Development. South Korea, Canada and Japan are the world leaders.

The new data showed African Americans and Hispanics gaining ground in college completion. From 2001 to last year, Hispanics rose from 4.4 per cent to 6.1 per cent of the nation's college-educated population. In the same span, blacks rose from 6.7 per cent to 7.6 per cent of all degree-holders.

Census data also showed that an associate degree in engineering or computing is worth, on average, as much as or more than a bachelor's in education or the liberal arts.

An associate degree in engineering yielded US$4,257 (S$5,354) in monthly earnings in 2009, compared with US$4,000 for a bachelor's in the liberal arts and US$3,417 for one in education.

A two-year degree in computing fetched US$4,000 a month, the same median earning as a four-year degree in the humanities.

Even a vocational certificate, a credential that generally requires months - not years - of school, can yield more future earnings than a bachelor's degree in a low-paying field. Employees with construction certificates earned US$4,904 a month in 2009, better than the median pay for a bachelor's in the humanities.

'So the point here,' Mr Bauman said, 'is that sometimes the subject a person has pursued is as important as how far he or she has gone in school.'

The data came from several new reports and was largely drawn from the American Community Survey and Population Survey.

Although certain fields pay well at any education level, the data suggests that going to school remains a shrewd investment.

The median monthly pay for a professional degree reached US$11,927 in 2009. That was more than twice the monthly pay for someone with a bachelor's degree: US$5,445. By contrast, a high school diploma was worth US$3,179 a month and an elementary school education yielded US$2,136 a month.

College-educated people were also less likely to lose their jobs during an economic downturn. Unemployment peaked at 17.9 per cent in early 2010 for those without a high school diploma - for those with bachelor's degrees, the highest unemployment rate was 5.9 per cent.

Engineering and science were the most common undergraduate courses, representing 34.9 per cent of the total.

Overall, the historic male advantage in higher education has nearly been erased. In 2001, men led by 3.9 percentage points in bachelor's degrees and 2.6 percentage points in graduate degrees. By last year, both gaps were down to 0.7 percentage point.

But women still 'earn less than men at every level of education', Mr Bauman said.

Men with advanced degrees earned almost 50 per cent more annually than women in 2009 - US$89,400 compared with US$61,500.