Eustat

Methodological Note: Linked series. Domestic trade index (ICI)

ICI. Linked series. Methodological Note

DOMESTIC TRADE INDEX (ICI) OF THE AUTONOMOUS COMMUNITY OF THE BASQUE COUNTRY. BASE 2005.

LINKED SERIES. ICI. Base 2000 – ICI. Base 2005

In May 2008, the results of the ICI, Domestic Trade Index, were published in Base 2005, by updating the series available, from 1993, existing until then in Base 2000.

The ICI is a weighted Laspeyres index that is based on calculated fixed weights with data from the base year. As it is a fixed base index, it becomes less representative as the time increases from the base period. It is therefore convenient to perform, after a certain period of time, what is known as the change of base, a process that breaks the continuity of the previously published series of the 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 until the moment when this change of base year takes place, and express all the series in the new calculation base.

In order to do so, the ICI index series base 2000 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 or another base for 2005:

The coefficients used for the link of the ICI series were obtained using the comparison by quotient of the data referring to 2005.

The main characteristics of the work carried out to proceed to the change of base can be summarised as:

• The coefficients were calculated as quotient of the annual averages in order to collect the greatest amount of information possible.
• Linked series of indexes in base 2000 were published, as that ensured series of indexes for 15 years, sufficient for any type of analysis, without considering the indexes obtained using previous bases.
• The linked series of indexes were published according to the Eustat standardised sectorial classification structure. In this specific case, sectors B and C of trade.