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MEXICO CITY, Aug 14 (Reuters) - Mexico's national statistics institute, INEGI, is preparing to introduce a new inflation methodology during the second half of next year that includes updated consumer spending patterns advocated by the International Monetary Fund, an official said.
The new system is based on an improved spending survey and will include revised weightings that factor in the spending patterns in small rural and urban centers, Fernando Pineda, an INEGI official involved in the redesign, told Reuters in an interview this week.
Pineda said INEGI's board would fix an exact date for the new index's launch during the fourth quarter, once it finishes evaluating the results of a public consultation over the changes held earlier this year.
"Tentatively, (the launch) could be during the second half of 2016," Pineda said.
Mexican inflation has cooled to a record low, staying below the central bank's 3 percent target for the last three months, and it is projected to remain below that level for the rest of the year.
However, a deep slump in the peso could fan price pressures higher next year back toward 4 percent, analysts said. The projected pace of those gains is crucial for the market to gauge how quickly Mexico's central bank could raise interest rates off a record low.
The new methodology will force economists to tweak their predictive models and their learning curve could increase the number of surprises for the market, when data comes out above or below market estimates, economists said.
Some of the key changes to the index were recommended by the IMF in late 2013, such as basing the weighting of products and services on a spending survey that covers an entire year and including spending data from towns with 15,000 people or fewer.
The current index is based on Mexico's main spending survey that is derived from poll results from one quarter, which means it may not account for some seasonal spending patterns, according to the IMF.
The updated spending information could remove a bias to overestimate the pace of some price changes, said Carlos Guerrero de Lizardi, an economist at the National Autonomous University of Mexico (UNAM).
The current index is based on older patterns of spending that do not take into account how consumers have substituted lower-cost products for more expensive ones, Guerrero said.
"They are creating a better measuring instrument and we should see a reduction in this upward bias," he said. (Editing by Matthew Lewis)