So oder so? : Zwei Maße für die Preisentwicklung im Außenhandel – Was lehrt uns der Vergleich?

Wie aussagekräftig sind die zwei Maße, die das nationale Statistikamt in Deutschland für die monatliche Preisentwicklung im Außenhandel bereitstellt? Die beiden Indizes, der so genannte Durchschnittswertindex und der Preisindex werden anhand ihrer spezifischen Charakteristika vorgestellt und miteinander verglichen.
The German Federal Statistical Office (FSO) is one of the few National Statistical Institutes that is capable of providing not only unit value indices (UVI) but also true price indices (PI) of both exports and imports on a regular (monthly) basis. This co-existence of two different measures of the price movement in foreign trade has, on occasion, caused some confusion and given rise to problems of interpretation. The paper examines in detail methodological differences between the two types of index numbers and tries to demonstrate empirically the effect of various aspects of index compilation on the observed time series of UVI and PI. The study was carried out with data from the Deutsche Bundesbank, who also took the initiative to investigate the issue. In the paper, there is a great emphasis on differentiating between two possibly conflicting sources of deviation between UVI and PI viz. the so-called “Laspeyres effect” and the effect of changes in the structure of traded goods. The paper also presents the very first published empirical assessment of the influence of quality adjustment on price indices by the FSO. Prior to quality adjustments, raw data are usually not published. At the request of the author, the price statistics department therefore kindly performed a special data analysis in the case of IT products. Ample evidence was found as regards the considerable smoothing and volatility reducing effect of quality adjustments, at least in these exemplary products. To sum up: although it may be difficult to demonstrate the effect of each methodological aspect by which the UVI and PI differ when taken in isolation, it is nonetheless clear that both types of indices may well be useful. The reason is that each of the two types of indices is satisfying one of the two, in a sense “polar” requirements of index numbers, requirements which are both equally sensible and yet difficult to reconcile. These requirements are to represent all currently relevant commodities on the one hand, and to permit a “pure” price comparison, undistorted by structural changes, on the other hand.
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