Robert M. Groves is leaving the Census Bureau for Georgetown University. (Tracy A. Woodward/The Washington Post)

Robert M. Groves was a transitional figure at the Census Bureau, which he headed for three years.

He presided over the 2010 Census, probably the last one to rely on paper questionnaires mailed through the U.S. Postal Service. And he was the first to write a blog (though he never got around to sending out tweets).

Now, as he prepares to leave the Census Bureau to become provost of Georgetown University this month, Groves is warning that the agency’s methods of collecting statistics are unsustainable.

“Because of the Constitution, the country will always have a census,” he said in an interview Friday at his office in the bureau’s Suitland headquarters, already stripped of his personal belongings. “But how we do the census and surveys will have to change.”

Cost is a big reason. Even though it came in $1.9 billion under budget, the last census cost $13 billion, about $42 a head. The price tag has doubled every decade since 1970. To cut costs, Groves closed six of the 12 regional census offices and oversaw early retirement offers that ushered out 200 veteran employees.

Groves says the United States will have to start making greater use of statistics already collected by private industry, as well as government. Not only will it save money, he said, but it will save time for people who are sent the detailed American Community Survey, which takes about 40 minutes to complete. It’s more efficient, he said, to reuse data that’s already been provided.

As Groves envisions it, the Census Bureau could mine credit card records for data on sales transactions. It could get data on income and poverty from tax returns and government records on Social Security, Medicare and food stamps. Statistics on real estate prices could come from Internet sites such as Zillow, and traffic cameras could be a source for estimates of commuting times.

Groves said everything would have to be guided by the same rules of anonymity that keep census records private.

“The mission of the Census Bureau is to describe society and the economic activity of the country,” he said. “We are completely uninterested in individuals.”

Some Americans already consider the census too intrusive. But Groves said that in other countries, people given the option were more concerned about giving up their time than their privacy.

In Canada, for example, residents are asked whether they prefer to answer the census questions on income or whether they will grant permission to look at their tax records.

“More than 80 percent say, ‘Look at the records,’ ” said Groves, one of the nation’s most prominent statisticians. “They’re more worried that their time is precious.”

Relying more heavily on “big data” sources does not mean the end of census forms sent by mail or some in-person interviews with people who don’t readily respond. To ensure accurate estimates, extra effort would be made to account for people who don’t use credit cards, for example, or otherwise take part in an economy easily charted through government and big business.

“We need to discuss these issues,” Groves said. “Our job is to present the country with the trade-offs, in money saved, the impact on quality and how people feel about it.”

Although it is too early to predict whether alternative options will be presented in the 2020 count, the Census Bureau plans to make the Internet an option for the first time, starting with American Community Survey (ACS) forms to be sent out in January.

Groves has spent a significant part of his tenure at the agency defending the ACS from critics who want to either eliminate it or make it voluntary.

“Talk to community leaders in states, or businesses,” Groves said of those who question its value. “Ask them how important it is in the lives of places trying to decide where to put retail establishments or hospitals or fire stations. All statistics are a trade-off of the value to the common good versus the burden they impose on people.”

If the ACS were not mandatory, Groves said, many people would toss the questionnaires out with the junk mail. Then it would cost tens of millions of dollars more to gather data accurate enough to be used for planning.

Groves, who is 63 and lives with his wife in Arlington, will spend his last day on the job on Friday. The following Monday, he starts at Georgetown.

Groves has said he’s an academic at heart and looks forward to heading back to a college campus.

Thomas L. Mesenbourg, the deputy director under Groves, will become acting director until a successor is chosen.

Groves said he believes the job is best filled by someone steeped in the world of statistics, as he was before being tapped by President Obama to head the bureau.

“I don’t think this is a job for political animals,” he said. “The most important thing to get right in this role is to make sure you’re serving in a nonpartisan, objective nature.”