How exactly should we choose an index number formula that will enable us to compare real outputs across countries? Index number comparisons between countries require information on the prices of products in the same units of measurement and information on expenditure shares by product category. The ICP gets expenditure shares using information on final demand expenditures from the national accounts of the participating countries. However, these national expenditure estimates are not very accu rate in some cases, and are not always constructed on a consistent basis across countries. It is very difficult or impossible to compare real outputs across countries at very different stages of development, since poor countries may consume products which are very different from the products consumed in rich countries (i.e., it is impossible to compare the incomparable).