Background Notes

Retail Sales Index

An updated Retail Sales Index (RSI) was introduced in the February 2009 release. The underlying methodology remains unchanged. The index is primarily intended to measure short-term trends.  For long term analysis allowance must be made for the fact that the series does not reflect changes in retail structures between updates.

Reference Period

The December 2015 period covers the 5 weeks from 29/11/15 to 02/01/16 and the January 2016 period covers the 4 weeks from 03/01/16 to 30/01/16.

Base Year

This set of monthly retail sales indices has been compiled with reference to base year 2005=100. The underlying structural weighting system is mainly based on the 2005 Annual Services Inquiry. Weights are updated from year to year using ratios of average weekly turnover by size class within business type.

Scope and Coverage

The Retail Sales Index covers, on a sample basis, the retail sales of retail businesses, wholesale businesses with sizeable retailing activity and the separate sales establishments of non-distribution enterprises. Excluded from the index coverage are the direct retail sales from non-distribution establishments together with the incidental retail sales of wholesale businesses. Also excluded are sales by hawkers, street stalls and other retailing activities not conducted from permanent business premises. The statistical unit used for reporting sales figures in this inquiry is the enterprise (i.e. complete business covering all branches).

Methodology

The business categories are based mainly on the Statistical Classification of Economic Activities in the European Community (NACE Rev. 2). The index system is structured on the retailing enterprises covered in the 2005 Annual Services Inquiry classified by 13 retail business categories and four turnover (excluding VAT) ranges.

The value indices are calculated by updating the aggregate 2005 retail turnover (including VAT) of these enterprises in these cells using the monthly retail sales data provided by the enterprise respondent panel from 2005 onwards.

The compilation of the index for a current month is based on the percentage change in average weekly sales in these cells over the corresponding monthly period of the previous year. The monthly volume indices are calculated by deflating the seasonally unadjusted value indices by specially constructed retail price indices derived from the Consumer Price Index (CPI). A copy of the Retail Sales Index methodology is available on our website (www.cso.ie).

Seasonal Adjustment

All Retail Sales “unadjusted” series are trading day adjusted as they are based on either a standardised four or five week reporting period (see reference period note above). Seasonal adjustment is conducted using the direct seasonal adjustment approach.  Under this approach each individual series is independently adjusted, e.g. aggregate series are adjusted without reference to the component series. Each individual seasonally adjusted series is calculated based on unadjusted data spanning from January 2000 to the current period.

The adjustments are completed by applying the X-12-ARIMA model, developed by the U.S. Census Bureau to the unadjusted data. This methodology estimates seasonal factors while also taking into consideration factors that impact on the quality of the seasonal adjustment such as:

• Calendar effects, e.g. the timing of Easter,
• The phase shift effect, i.e. the fact the reporting period of the RSI does not coincide with the calendar month and
• Outliers, temporary changes and level shifts in the series.
   
  Note: Athough each series is independently adjusted, no changes were made to the Motor Trade series Seasonal Adjustment model to take account of the introduction of the new car registration system in Ireland in 2013.  This is due to a lack of historical data or comparable models that would be required to inform the Seasonal Adjustment model. Once a seasonal pattern has been established we will be in a position to review the Motor Trade Seasonal Ajustment model.
   
  Also to note, the Motor Trade group comprises all retail motor trade activities, of which new car sales accounts for approximately 45% of turnover.

For additional information on the use of X-12-ARIMA see (Findley, D.F., B.C. Monsell, W.R. Bell, M.C. Otto, and B. Chen (1998), “New Capabilities and Methods of the X-12-Arima Seasonal Adjustment Program”, Journal of Business & Economic Statistics, 16, pp. 127-177.) and  www.census.gov/srd/www/x12a/

Reference

A detailed description of the Retail Sales Index is available on the CSO website  http://www.cso.ie/en/surveysandmethodology/services/retailsalesindexsurveys/retailsalesindex/ Retrospective details for the current and previous series are also available on the CSO website.

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