text: selected. The Group is of the opinion that it has solved the problem in an appropriate way, but it has not had the time to deal with this complex matter in all its aspects. Further consideration of the problem might therefore help to arrive at better and more reliable results. 128. Although the construction of price indexes and PPPS for the category of operations and maintenance did not seem to present any major difficulties different from those existing in the civilian sector, the Group felt that its questionnaire should have included a larger number of items. This would probably have contributed to an improvement of the results. 129. For the category of procurement, it has not been possible for the Group to COnStrUCt price indexes for all participating States owing to a lack of data. Not only are data for this category generally scarce, but the particular nature of weapon systems and other items of military equipment often present Special difficulties in the comparison of such items and of their prices. 130. One particular problem has to do with the calculation of unit prices which in most cases depend on several different factors such as the production cycle and the imputation of fixed costs, as described in chapter II. Another problem, as already noted above, is that of handling changes in quality over time. All these factors constitute difficulties not only with regard to the construction of price indexes but also with regard to PPPs. The Group has tried its best to overcome these difficulties but is, at the same time, conscious of the fact that it may not have found in all cases the most appropriate solutions. It would, for instance, be worth considering the use of methods by which so-called theoretical prices are estimated with the help of regression analysis relating the price of an item to its 
analysis: The text discusses the construction of price indexes and purchasing power parities (PPPs) for various categories, including operations and maintenance, procurement, and military equipment. It highlights the challenges and difficulties encountered in this process, such as limited data, complexities in calculating unit prices, and handling changes in quality over time. The text does not explicitly mention any specific sector or industry, but rather focuses on the methodological aspects of constructing these economic indicators.