China will revise the estimated size of its economy by 3% in 2013, even as weak factory numbers released Tuesday showed the world’s second-largest economy continues to struggle.

The revision, which is lower than many economists had expected, will change little in practice and likely won’t significantly affect previously reported economic growth rates.

Still, it could push up the date when the size of China’s economy officially surpasses the U.S. economy and influence the debate about the size of China’s imbalances, debt and slowdown. It could also impact how China counts growth in coming periods–which is being closely watched as the nation struggles to hit its growth target of about 7.5% this year.

Reflecting decades of central planning, China has long undercounted sectors such as services, how rent is valued and research and development. Capturing this data has become increasingly important as Beijing tries to rebalance its economy away from investment and toward services and consumer spending.

At a news conference Tuesday, National Bureau of Statistics chief Ma Jiantang said China would revise the size of its GDP by 3% to reflect more detailed information resulting from China’s 2013 census.

Mr. Ma didn’t say whether the revision was up or down, but China is striving to capture more economic activity in the data that it may have missed before and past revisions have all resulted in substantial increases. Mr. Ma said the bureau will offer more details on Friday.

Any upward revision could speed up the point when China overtakes the U.S. as the world’s largest economy, a position the U.S. has held since it surpassed Britain in 1872. The International Monetary Fund reported this year that China’s economy has surpassed the U.S. in purchasing power parity terms, a measure of the relative cost of goods in each country. Before the revision, economists expected China to take the lead in absolute terms some time after 2020, fueled by a rapid increase in consumer spending.

To some extent these shifts remain largely symbolic, however. China has four times the population of the U.S., so on a per-person basis Americans will remain wealthier on average for the foreseeable future.

The change in how China’s GDP is calculated is expected to fuel the debate over the reliability of Chinese statistics and to what extent the recalculation helps China meet its own goals as it struggles to hit its growth target.

The expected change is based on methodology crafted several years ago by the United Nations and other organizations that other countries, including the U.S., adopted years ago. “They are belatedly doing what the rest of the world has been doing all along,” said Ramesh Chander, an economist and former World Bank adviser.

The changes aren’t expected to end head scratching over Chinese statistics, epitomized by the mismatch between China’s national GDP figure and the weighted sum of provincial GDP. “There are plenty of conspiracy theories out there about what determines China’s GDP figure,” said Mark Williams, economist with Capital Economics. “But at least in this case it’s just China getting up to date with the methodology.”

Research provider Rhodium Group suggested in a research note that a more accurate accounting of services and consumption could chip away at the political clout of traditional industries and strengthen the case for reform. “The old-line growth model with diminishing returns looks all the more wasteful in light of the renewed GDP figures,” it said. The change could also alter the budgets that various interest groups such as schools or the military receive, to the extent these are calculated as a percentage of GDP.

While the change should help improve transparency and improve the data going into economic decisions, the apparent size of China’s economy on a comparative basis will remain distorted, some analysts said, until there is a better accounting of its bad debt.

“You must believe that GDP is overstated by the failure to write down bad loans,” said Peking University Guanghua School of Management professor Michael Pettis.

Also on Tuesday, HSBC Holdings PLC and data provider Markit reported that its preliminary purchasing managers index fell to 49.5, a seven-month low, down from 50.0 in December. A reading above 50 indicates expansion while a number below that signals contraction. Domestic demand slowed considerably as did price pressure, renewing calls for a more accommodative policy. “It’s still pretty weak,” said CIMB economist Fan Zhang. “This clearly suggests they have to do more easing and more fiscal support.”

Separately, the Ministry of Commerce reported Tuesday that China attracted $10.36 billion in foreign direct investment in November, up 22.2% year on year, compared with $8.53 billion in October, a 1.3% year-over-year increase.

Thursday’s manufacturing numbers are the latest in a string of weaker indicators in recent weeks. GDP growth slowed to 7.3% on year in the third quarter, its slowest pace in more than five years, amid slower investment growth, industrial production and a worsening property slump in November.