except for special difficulties with regard to the availability and auality of information and some other particularities which are dealt with in section D below. 28. Thus, the choice of products whose prices and weights are used for the construction of a price index is often subjective, as the availability of prices and quantity data (or expenditures) have to be considered and not only the statistical representativeness. If representative items of expenditure are excluded from the price index, the real movement of prices would be different from that described by the price index. / . . . A/40/421 English Page 13 29. There is no problem of comparison as long as the products retain their characteristics. Changes, however, are likely to appear when comparisons are made for long periods. As shown below, several methods may be used to take into account changes in the qualities of products from one period to another. 30. It is sometimes useful to calculate a fictitious base price, in other words, the price which the new product would have had if it had existed during the base period. To do this, a special characteristic of the product susceptible Of explaining its price must be determined. This supposes a direct and proportional relationship between the price of a product and its main characteristics. However, such relationships are often not clearly established. Moreover, the quality of a product is judged differently by different users. 31. In the econometric method, regression techniques are used to establish a relationship between the price of a product and several of its main characteristics. In this manner, a fictitious price can be calculated. However, the regression coefficients are often difficult to interpret and the relationships are usually unstable over time. 
**Analysis**
The text appears to be discussing the construction and methodology of price indices, particularly in the context of economic statistics. It touches on the challenges of accounting for changes in product quality and characteristics over time, and the use of econometric methods to establish relationships between product prices and characteristics. The text does not seem to be focused on any specific sector or industry, but rather on the broader economic concept of price indices.

**Category**
Economy, Industry