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Methodological Note: Linked series. Large Stores Trade Index (ICGS)

ICGS. Linked series. Methodological Note

LARGE STORES TRADE INDEX (ICGS) OF THE AUTONOMOUS COMMUNITY OF THE BASQUE COUNTRY. BASE 2005.

LINKED SERIES. ICGS. Base 2004 – ICGS. Base 2005

In May 2008, the results of the Large Stores Trade Index (ICGS) were published in Base 2005. In November 2005, this index underwent a change of base, on that occasion 2004, and as had occurred with base 2000 and base 1991, the year in which it began to be published.

We now present the series of indices obtained according to the new base 2005 with the series of indexes previously obtained using previous bases.

The process to change the base year for the ICGS represents a break in the continuity of the series of the previously published index. As a result, a series of continuous data in time is required to be used both for calculating predictions and analysis of the historic development of the index. It is necessary to determine a process that enables us to link the series of the index calculated to date in which this change of base year takes place, and express all the series in the new calculation base.

In order to do so, the ICGS index series base 2004 are transformed into index series with base 2005 multiplied by their corresponding "link coefficients". These "k" link coefficients are calculated as the quotient of the average annual value of indexes expressed in one and another base for 2005:

The coefficients used for the link of the ICGS series were obtained from the comparison by quotient of the data referring to 2004.