Aggregation over different qualities: Are there generic commodities?

Aggregation over goods is always an issue in empirical analysis. However, while the problem of aggregation over different goods often is analysed, little attention has been given to the properties of generic goods such as e.g. coffee, wheat and salmon. Often these generic commodities contain a number of different qualities. In this paper a test for aggregation based on Lewbel's Generalized Composite Commodity Theorem (GCCT) using only price data is used to validate aggregation. Weshow by using price series for different weight-classes of salmon that the generic term salmon can be used for these products