Although the lack of sufficiently detailed data makes it difficult to apply a common definition of military expenditure on a worldwide basis, SIPRI has adopted a definition as a guideline. Where possible, SIPRI military expenditure data includes all current and capital expenditure on: (a) the armed forces, including peacekeeping forces; (b) defense ministries and other government agencies engaged in defense projects; (c) paramilitary forces, when judged to be trained and equipped for military operations; and (d) military space activities. This should include expenditure on: i. personnel, including: a. salaries of military and civil personnel; b. retirement pensions of military personnel, and; c. social services for personnel; ii. operations and maintenance; iii. procurement; iv. military research and development (R&D); v. military infrastructure spending, including military bases; and vi. military aid (in the military expenditure of the donor country). SIPRI’s estimate of military aid includes financial contributions, training and operational costs, replacement costs of the military equipment stocks donated to recipients and payments to procure additional military equipment for the recipient. However, it does not include the estimated value of military equipment stocks donated. Civil defense and current expenditures on previous military activities, such as veterans' benefits, demobilization, conversion and weapon destruction are excluded. In practice it is not possible to apply this definition for all countries, and in many cases SIPRI is confined to using the national data provided. Priority is then given to the choice of a uniform definition over time for each country in order to achieve consistency over time, rather than to adjusting the figures for single years according to a common definition. In the light of these difficulties, military expenditure data is most appropriately used for comparisons over time, and may be less suitable for close comparison between individual countries. Reference should always be made, when comparing data for different countries, to the footnotes and special notes attached to the data for these countries, which indicate deviations from the SIPRI definition, where these are known.