Representing a historic high, three in 10 adult Americans held bachelor’s degrees in 2011, census officials reported Thursday.

College attainment has crept upward, slowly but steadily. In 1947, just 5 percent of Americans 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 Kurt Bauman, chief of the Census Bureau’s Education and Social Stratification Branch, during a telephone news conference to announce the data.

The Washington region remains the nation’s best-educated metropolis. As of 2010, 46.8 percent 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 percent college attainment.

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

The new data show African Americans and Hispanics gaining ground in college completion. From 2001 to 2011, Hispanics rose from 4.4 percent to 6.1 percent of the nation’s college-educated population. In the same span, blacks rose from 6.7 percent to 7.6 percent of all degree-holders.

But in terms of future earnings, education level matters less these days than in previous generations, and field of study matters more.

Census data show that an associate’s degree in engineering or computers is worth as much or more, on average, than a bachelor’s in education or the liberal arts.

An associate’s degree in engineering yielded $4,257 in monthly earnings in 2009, compared with $4,000 for a bachelor’s in the liberal arts and $3,417 for a bachelor’s in education.

A two-year degree in computers fetched $4,000 a month, the same median earnings 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 $4,904 a month in 2009, better than the median pay for a bachelor’s in the humanities.

“So the point here,” Bauman said, “is that sometimes a subject a person has pursued is as important as how far they went in school.”

The data come from several new reports and are largely drawn from the American Community Survey and Current Population Survey.

Although certain fields pay well at any education level, the data suggest that going to school remains a shrewd investment. Median monthly pay for a professional degree reached $11,927 in 2009. That was more than twice the monthly pay for someone with a bachelor’s degree: $5,445. By contrast, a high school diploma was worth $3,179 a month, and an elementary school education yielded $2,136 a month.

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

Women still “earn less than men at every level of education,” Bauman said. Men with advanced degrees earned almost 50 percent more annually than women in 2009: $89,400 compared with $61,500.