The SIPRI Military Expenditure Database consistent time series on the military spending of 172 countries since 1988. Figures in a share of GDP are present according to calendar year, calculated on the assumption that, where financial years do not correspond to calendar years, spending is distributed evenly through the year. SIPRI military expenditure data is based on open sources only, including a SIPRI questionnaire which is sent out annually to all countries included in the database. The collected data is processed to achieve consistent time series which are, as far as possible, in accordance with the SIPRI definition of military expenditure, detailed in the sources and methods. The main purpose of the data on military expenditure is to provide an easily identifiable measure of the scale of resources absorbed by the military. Military expenditure is an input measure which is not directly related to the 'output' of military activities, such as military capability or military security. Long-term trends in military expenditure and sudden changes in trend may be signs of a change in military output, but such interpretations should be made with caution. Military expenditure data measured in constant dollars are a trend indicator of the volume of resources used for military activities, which allow comparisons to be made over time for individual countries and between countries. The share of gross domestic product (GDP) is a rough indicator of the proportion of national resources used for military activities, and therefore of the economic burden imposed on the national economy.