Preface approaches proved a successful competitor to the X-ll variant, partly because revisions of seasonal factors tended to be larger.Tn 1965, we certainly did not expect that the X-ll variant of Method TT would still be playing an important role in seasonal adjustment 35 years later. It also seemed likely that an approach to measuring the standard errors in the seasonal factors in X-ll would be developed. The authors are to be congratulated for an up-to-date review of the X-ll Method. Perhaps this review will not only help further extend the life of X-ll, but will help in the development and widespread use of new methods that bear little resemblance to those of the past.

High frequency data, i.e. data observed at infra-monthly intervals, have been used for decades by statisticians and econometricians in the financial and industrial worlds. Weekly data were already used in the 20's by official statisticians to assess the short-term evolution of the Economy. For example, Crum (1927) studied the series of weekly bank debits outside New York city from 1919 to 1026 and proposed a method to seasonally adjust these data based on the median-link-relative method developed by Persons (1919).

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SummaryMost of the Euro-zone economic short-term indicators are computed through aggregation from Member States data. The seasonally adjusted figures can be calculated by seasonally adjusting the aggregate (direct approach) or aggregating the seasonally adjusted national data (indirect approach). Statistical and practical considerations to choose the right strategy are given in the paper. An application to the Euro-zone GDP is presented. The same aggregation problem encountered in the case of seasonal adjustment will persist when extracting the business cycle. Moreover, since raw figures imply problems in terms of excessive noise of the series, analysts generally prefer the use of seasonally adjusted time series. As a consequence, the problem of choosing between direct and indirect both in seasonal adjustment and in business cycle extraction appears to be closely linked. In fact, the approach chosen to seasonally adjust the data can in theory lead to different results when the cyclical component has to be extracted from seasonally adjusted data. After a review of different filters widely used in the literature, we extracted the cycle indicator for the Euro-zone employing the Baxter-King filter to data coming from both direct and indirect seasonally adjustment approach and then compared the relative results.