* China Momentum Indicator: http://link.reuters.com/tym35w
* China rail freight and GDP: http://link.reuters.com/fyq32t
* China monetary policy and GDP: http://link.reuters.com/vyt35s
By Ross Finley
LONDON, Aug 6 (Reuters) - China's economy is growing only half as fast as official data shows, or maybe even slower, according to foreign investors and analysts who increasingly challenge how the world's second largest economy can be measured so swiftly and precisely.
Beijing's official statisticians reported last month that China's economy grew by a steady 7.0 percent in the first two quarters of the year, spot on its official 2015 target.
That statistical stability comes at a time when prices of global commodities, which China still hungers for despite a campaign to rebalance the economy away from investment and manufacturing toward consumer spending, have cratered.
But perhaps the biggest question is how a developing country of 1.4 billion people can publish its quarterly gross domestic product (GDP) statistics weeks before first drafts from developed economies like the United States, the euro zone or Britain, and then barely revise them later.
"We think the numbers are fantasy," said Erik Britton of Fathom Consulting, a London-based independent research firm and one of the more vocal critics of official Chinese data. "There is no way those numbers are even close to the truth."
The uncanny official calm in China GDP data may well be contributing to sceptics' exit from Chinese assets just as the authorities struggle to manage a volatile stock market.
Fathom, which decided last year to stop publishing forecasts of the official GDP release and instead publish what it thinks is really happening, reckons growth will be 2.8 percent this year, slowing to just 1.0 percent next year.
One issue is that so many other forecasters stick to the script. In the latest Reuters poll of mainly sell-side bank economists, based both inside and outside China, the range of opinion is 6.5-7.2 percent. For next year, it's 6.3-7.5 percent.
"MAN MADE"
Li Keqiang, now Chinese Premier, was cited in leaked U.S. diplomatic cables years ago from when he was Communist Party head in Liaoning province calling GDP figures "man-made" and unreliable. This remains a buttress for widespread scepticism.
Fathom publishes a simple indicator based on three variables that Li said at the time he watched for a better view of how his local economy, and by extension the national one, was faring: electricity consumption, rail cargo volume, and bank lending.
That implies a growth rate of 3.2 percent, and shows a significant decoupling from the official rate since late 2013 based on a plunge in rail freight volumes and below-trend growth in electricity production.