The imprecision due to sampling variability, which is measured by the relative standard error, should not be confused with inaccuracies that may occur because of inadequacies in the source of information, imperfections in reporting by respondents, and errors made in the coding and processing of data. Inaccuracies of this kind are referred to as non-sampling error, and may occur in any enumeration whether it be a full count or only a sample. Every effort is made to reduce the non-sampling error to a minimum by the careful design of questionnaires, efforts to obtain responses for all selected organisations, and efficient operating procedures. As happens with all seasonally adjusted series, the seasonal factors are reviewed annually to take account of each additional years data. The results of the latest review are included in this issue.
Information regarding the methods used in seasonally adjusting the series, is available on request.
Since seasonally adjusted statistics reflect both irregular and trend movements, an upward or downward movement in a seasonally adjusted series does not necessarily indicate a change of trend. Particular care should therefore be taken in interpreting individual quarter to quarter movements. While current price estimates of the value of work done reflect both price and volume changes, chain volume estimates measure changes in value after the direct effects of price changes have been eliminated and therefore only reflect volume changes. The deflators used to revalue the current price estimates in this publication are derived from the same price data underlying the deflators compiled for the new engineering construction component of the national accounts aggregate 'Gross fixed capital formation'. The reference year will be updated annually in the June quarter publication.