* Data still lags private estimates, undermines credibility
* Investors need accurate price data to evaluate risks
BUENOS AIRES, May 7 (Reuters) - When Argentina revamped its consumer price index this year investors were relieved that Latin America's No. 3 economy was trying to win back the trust of financial markets after years of blatantly underreporting inflation.
Shunned by the global bond market for more than a decade, Argentina unveiled a new consumer price index (CPI) as part of a push to reestablish international credit and bolster central bank reserves after a more than 30 percent fall last year.
But the new index continues to clock inflation at well below analysts' estimates and the government has stopped listing the products measured, raising questions over how much the data is being dragged down by price controls that the administration puts on scores of food and household items.
"We are not sure how far the deformation of data goes because the government does not publish a list of the 520 products included in the index, so we do not know how many of those products are subject to price caps," said economist and former central bank chief Rodolfo Rossi.
From 2007 through 2013 Argentina's CPI reflected little more than the opinion of the government's former price czar Guillermo Moreno. A pugnacious defender of President Cristina Fernandez's inflationary fiscal policies, Moreno was known for trying to strong-arm retailers into keeping prices down.
In those years, Argentina reported inflation at about half the rate estimated privately. The discrepancy hurt confidence in the government, widened Argentina's sovereign risk spreads and got the country censured by the International Monetary Fund.
The official inflation rate last year was 10.9 percent versus private sector estimates of about 25 percent.
The new index showed inflation of 3.7 percent in January, way under the 5.6 percent estimated by analysts in a Reuters poll. In February, the government clocked inflation at 3.4 percent versus 4.2 estimated by economists surveyed by Reuters.
In March the official rate of 2.6 percent was closer to the 3.0 percent in the Reuters survey and the 3.3 percent offered in a separate poll published by Argentina's Congress.
Economy Minister Axel Kicillof says only the state has the capacity to gauge inflation in a country as big as Argentina.
'A BLACK BOX'
The Indec National Statistics Agency provides no data breakdown for the six regions that make up the CPI. It does not publish average prices for the items included, nor does it publish core inflation, which measures long run trends by excluding transitory price changes.
"The CPI will lack credibility as long as these details cannot be seen by the public," Marcela Almeida, an Indec mathematician, told Reuters.
"Starting in January we don't know what's being measured because they reveal less and less information," she said.
Almeida had some of her duties taken from her by her managers in late 2007 after her first public complaints about the CPI. She was reinstated by a judge this year but is no longer directly involved in preparing the CPI.
Indec officials declined interview requests.
Fernandez's second and final term as president ends in late 2015. In the past three months, she has cut heating gas subsidies, let the currency devalue and is about to start debt renegotiation talks with the Paris Club of creditor nations.
Investors hope the moves will improve Argentina's finances but are still concerned about the accuracy of government data.
The CPI data is presented every month as "a black box", said Indec employee and union spokesman Raul Llaneza. "You don't know what goes into it," he said. "The nucleus of the problem is they are hiding information needed as a tool to analyze the economy."
(Additional reporting by Anna Yukhananov in Washington; Editing by Kieran Murray and Andrew Hay)