GOVERNMENT statisticians shun the limelight, which only ever finds them when things go awry. So it is with India’s national bean counters, who are struggling to convince the world that an economy with idle factories, sagging exports and ailing banks grew by 7.5% in 2015, as their models purport to show. Ever since a new methodology for calculating GDP was adopted last year, India has appeared to be the world’s fastest-growing big economy, outpacing China. But scepticism about the data is growing even faster. Growth figures are calculated by first arriving at the value of economic output over a given period and then comparing it with the prior period. The difference between the two gives a nominal rate of growth (ie, without any adjustment for inflation). Most observers agree that India’s egg-heads perform these tasks well. The problem seems to be the “GDP deflator”, a gauge of inflation by which the data are adjusted to derive the “real” growth rate. The higher inflation is assumed to be, the bigger the slice of nominal growth that is attributed to price rises rather than genuine increases in output.

Mercifully enough, GDP deflators do not normally attract much attention. Typically, different bits of the economy are deflated by whichever inflation series is most apposite. India compiles two measures: a wholesale price index (WPI), measured at the factory gate, and a consumer price index (CPI), which tracks how much consumers pay. Changes in the two usually move in tandem, so it doesn’t much matter which is used.

All that has changed since prices of oil and other commodities tumbled last year, causing wholesale prices to decline. Despite this, deflation remains a distant dream for shoppers: the price of consumer staples is still rising by over 5% a year. The gap between the change in the two indices swelled from nothing to nine percentage points in September, before falling back to six percentage points. The statisticians use WPI to deflate the nominal growth of service output, which accounts for roughly half the economy, even though most services have not benefited much from low commodity prices. The blended inflation figure used to deflate the nominal data may therefore be too low, making real GDP growth come out too high (see chart).

Investors, at any rate, roundly disbelieve India’s growth figures. Nevsky Capital, a hedge fund, cited dodgy data from India, among other places, as a reason to shut up shop at the start of the year. Even the government’s own chief economic adviser has admitted he is sometimes flummoxed by the data. A cottage industry has sprung up to cater to the sceptics, blending various indicators of economic activity to produce new gauges of growth.

Such home-brewed statistics have been common in China for some time: Li Keqiang, now the country’s premier, admitted as a provincial governor that he all but ignored “man-made” economic statistics in favour of hard-to-fiddle data such as railway-cargo volumes, electricity consumption and loans made by banks. The Economist began publishing a “Keqiang Index” when his habits became known in 2010.

Ambit Capital, a broker based in Mumbai, now computes its own “Keqiang Index” for India, which implies a real growth rate of 5.4%. Economists at HSBC, a bank, think 5.9-6% is closer to the truth.

If the divergence between WPI and CPI is indeed distorting the data, its nefarious impact should soon disappear as year-on-year readings of commodity prices stabilise and so reunite the two series. Statistical improvements are also promised by India’s boffins. But measuring fast-evolving economies is tricky: Nigeria two years ago announced its GDP was almost twice as big as official statistics had previously indicated. It does not help that 90% of India’s workers toil in the informal sector.

“The debate will reduce but not go away,” predicts Pronab Sen of the National Statistical Commission, an advisory body. Though they have flaws, India’s official statistics are pretty good by emerging-market standards, he argues. But in a global economy with few bright spots, where only America and China are adding a bigger amount to global GDP, it would be comforting to be more certain.