As growth in the world’s second-largest economy slows, the spotlight has intensified over the accuracy of China’s growth figures. This week, Xu Dianqing, an economics professor at Beijing Normal University and the University of Western Ontario, joined the debate with an estimate that China’s gross domestic product growth rate might just be between 4.3% and 5.2%.

China’s official growth rate in 2015 was 6.9%, the slowest pace in more than two decades, allowing the government to hit its target of around 7%. But longstanding questions over China’s statistical methodology have spurred a cottage industry in alternate growth indicators.

Many of these analyze other measures believed to be less subject to political pressure in estimating actual growth, including indices compiled by economists at Capital Economics, Barclays Bank, the Conference Board and Oxford Economics. Most peg China’s annual growth in the 4% to 6% range.

Mr. Xu told reporters at a briefing this week that the focus of his concern is the growth rate for China’s manufacturing sector, which according to official figures grew 6.0% last year and accounts for 40.5% of the economy.

A closer look at underlying indicators, however, including thermal power generation, railway freight volume, and output from the iron ore, plate glass, cement and steel industries released monthly by the National Bureau of Statistics paint a different picture, he said.

Of some 60 major industrial products, nearly half saw output contract in the January to November period, while railway cargo volume fell 11.9% for all of last year, according to official sources.

Given weaker industrial output in China and more than three years of industrial deflation, a 6% expansion for manufacturing in 2015 is questionable “no matter how the number is counted,” said Mr. Xu, who added that he believes it’s more probable that industry and construction grew at most by 2% last year and perhaps not at all.

That translates into economic growth that tops out at 5.2% last year and perhaps something in the 4s, assuming the official agriculture and service sector growth figures are correct, he said.

Mr. Xu said it’s unlikely that the service sector– sometimes cited as an explanation for growth rate discrepancies – did better than reported by authorities.

Many attempts to parse China’s growth rates take inspiration from the nation’s top economic official, Premier Li Keqiang, who as a provincial governor in Liaoning was quoted in a leaked U.S. diplomatic cable saying that China’s GDP numbers were “man-made” and therefore unreliable. Mr. Li recommended looking at three indices—electricity consumption, rail cargo volume, and the volume of loans disbursed – for a more accurate picture. This and subsequent variations became known as the “Keqiang Index.”

In December, state media said that “water injection” -- a Chinese phrase for cooking the books – by local officials in the northeast rustbelt provinces of Liaoning, Heilongjiang, and Jilin – had distorted investment and growth figures for years, without providing details.

The National Statistics Bureau did not reply to a request for comment. In a news briefing last week after China’s 2015 growth figure was released, the agency’s head Wang Baoan told reporters that China’s economic data was “valid and reliable” and its methodology in line with “global standards.”

The statistics agency has some 20,000 people in an affiliated investigation team that have studied, verified and “improved the production quality of the statistics,” he added.

Mr. Wang could not be reached for comment. On Tuesday, China’s antigraft agency said on its website that it was investigating Mr. Wang for alleged violations of discipline, a term typically used by investigators for corruption charges. There is no suggestion that the probe is linked to his duties as head of the statistics agency or to its statistical methodology.

Mr. Xu said his calculations and his conclusion that growth is slower than the official rate suggests assume that the official figures for growth in the agriculture and service sectors – which are not released in detail -- are correct. If the 8.3% increase for the service sector and the 3.9% increase for agriculture are questionable, actual growth could be lower, he added.

--Pei Li. Follow him on Twitter @teamlipei.