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Changes to Retail Trade Survey seasonal adjustment

As advised in Retail Trade Survey: June 2014 quarter, we are making changes to the Retail Trade Survey (RTS) seasonal adjustment from the September 2014 quarter (to be released on 17 November 2014).

Note that the following data is not affected by the changes explained in this article: 

  • quarterly Gross Domestic Product data 
  • actual Retail Trade Survey data.

Download revised backseries

To be ready for the release of retail trade data on 17 November 2014, you can download backdata that incorporates these changes up to the June 2014 quarter from: 

  • Infoshare 
    Subject category: Industry sectors
    Group: Retail Trade (ANZSIC06) – RTT 
  • Seasonal adjustment backseries for Retail Trade Survey: June 2014 quarter (Excel, 7 sheets) from the downloads box on this page.

Our new seasonal adjustment tool – X-13ARIMA-SEATS

We will be using the X-13ARIMA-SEATS seasonal adjustment tool for the RTS from the September 2014 quarter.

The seasonal adjustment tool we previously used for the RTS (X-12-ARIMA) is no longer supported by its designers; the US Census Bureau. The change to X-13ARIMA-SEATS ensures continued support, includes improved functionality, and is in line with international best practice.

X-13ARIMA-SEATS uses the same general methodology as X-12-ARIMA. However, there are some minor improvements to the way trading day effects are accounted for, and how the weights (ie measure of importance) are calculated for extreme values. These methodology improvements have resulted in small changes to the previously published RTS series.

We have also taken this opportunity to apply a leap-year adjustment to the seasonal adjustment of RTS sales. As a result, the levels of sales for March quarters during leap years (eg March 2012 quarter) have been revised downwards.

The change to X-13ARIMA-SEATS affects all seasonally adjusted and trend series for the RTS, including sales value (by industry and region), sales volume, and stock.

New seasonally adjusted vehicle-related series

We will also be introducing new seasonally adjusted vehicle-related series from the release on 17 November 2014.

Historically, sales for the two vehicle-related retail industries (ANZSIC06 motor vehicles and parts retailing; and fuel retailing) have not shown reliable seasonal patterns, and have therefore not been seasonally adjusted.

However, seasonal patterns have emerged in recent years, which now warrant adjustment. These are: 

  • Motor vehicle and parts retailing
    A seasonal pattern has emerged in motor vehicle and parts sales values and volumes since mid-2009. Seasonal adjustment will be applied from the June 2009 quarter onwards for both the sales value and volume series. The earlier portion of these series (from the September 1995 quarter to the March 2009 quarter) will remain unadjusted, as it does not display a seasonal pattern. 
  • Fuel retailing
    A seasonal pattern has emerged in the value of fuel retailing sales since mid-2009. Seasonal adjustment will be applied from the June 2009 quarter onwards for sales values. The earlier portion of this series (from the September 1995 quarter to the March 2009 quarter) will remain unadjusted, as it does not display a seasonal pattern.

    A seasonal pattern has been identified in the volume of fuel sales. Seasonal adjustment will now be applied for the full length of this series, from the September 1995 quarter.

These new adjustments will feed through to the seasonally adjusted retail sales total, and respective trend series. The vehicle-related stocks series will remain un-seasonally adjusted. 

Graphs showing the effect of the new seasonal adjustments

The following graphs show the effect the new seasonal adjustments (arising from the new tool and the new vehicle-related series) have had on total retail sales values and volumes, and on the vehicle-related series.

The split period line on the graphs marks the point at which the new vehicle-related seasonal adjustment begins.

Seasonally adjusted total retail sales values, quarterly, June 2008 to 2014

Seasonally adjusted total retail sales volumes, quarterly, June 2008 to 2014

Motor vehicle and parts sales values, quarterly, June 2008 to 2014

Motor vehicle and parts sales volumes, quarterly, June 2008 to 2014

Fuel sales values, quarterly, June 2008 to 2014

Fuel sales volumes, quarterly, June 2008 to 2014

For more information contact:
Kate Jackett or Tehseen Islam
Christchurch 03 964 8700
Email: info@stats.govt.nz

Page published 31 October 2014

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