errors which result from comparisons by means of exchange rates. It involves collecting information on prices and characteristics for a selected number of goods and services (items) as well as on quantities and/or expenditures, The selected items should be correctly defined, common to many countries, representative of the subcategories analysed and economically significant. States should therefore be asked to furnish the prices (or quantities) of similar goods and services. 39. The fundamental principle is to include the goods and services with the largest expenditure weights. They should, however, also be representative of their respective subcategory. In the case of a subcategory where price patterns are uniform among all countries, a single product can be selected. On the other hand, / . . l A/40/421 English Page 15 if the relative prices of the products within a particular subcategory vary widely from country to country, a large sample of significant goods and services should be chosen. (This could be a critical problem with ren rd to military expenditures, owing to the relatively poor quality and availability of data.) It is usually impossible to make an inventory of identical goods and services in the countries under study. Some items of equipment may not exist in some countries or be present in very small numbers. One should therefore seek to choose products which are available in all countries and avoid comparing items which are of greatly differing importance in participating countries. 40. The selection of prices, is another difficult matter giving rise to several auestions. For example, should gross or net salaries be selected? How are differences in social legislation to be taken into account? Should the sale price or the cost price of products be used? How can opportunity costs be taken into account when the price of an item is clearly subsidized or otherwise inappropriate? 
**Analysis**
The text appears to be discussing the methodology for comparing and analyzing economic data across different countries, with a focus on the selection of goods and services to be used in the comparison. The text highlights the challenges of ensuring that the selected items are representative and comparable across countries, and discusses the need to consider factors such as differences in social legislation and the availability of data. Given the focus on economic data and the challenges of comparing data across countries, it seems likely that the text is related to international economic statistics.

**Category**
International trade