The Extraordinary Annual Seasonal Review undertaken this quarter has individually assessed each series to determine how each observation from the previous year should be treated for estimation of the seasonal factors used to seasonally adjust the current end of the series. This process has ensured disruption to series caused by COVID-19 does not unduly affect estimates of the seasonal factors.
For series which have used the forward factors method, revisions to the seasonally adjusted series are likely to be larger than the revisions historically observed each quarter when concurrent adjustment was used. Series which have used forward factors have used static seasonal factor estimates for the previous year, and the review has updated these seasonal factor estimates by incorporating relevant information from the previous year's observations. When concurrent adjustment is used, revisions to the seasonally adjusted estimates will typically be smaller because seasonal factor estimates are updated quarterly on the basis of the addition of just one extra data point.
A decision was made about the future use of forward factors/concurrent adjustment. In the case where the forward factors method will continue to be used, new forward factors have been calculated.