The most comprehensive estimates, for China and Russia, have been presented in detail in previous Yearbooks. In the second case, differing time series are linked together. In order not to introduce assumptions into the military expenditure statistics, estimates are always based on empirical evidence and never based on assumptions nor extrapolations. Thus, no estimates are made for countries which do not release any official data, but these countries are displayed without figures. Round brackets are used when data are uncertain for other reasons, such as the reliability of the source or because of the economic context.
Figures are more unreliable when inflation is rapid and unpredictable.
Supplementary allocations made during the course of the year to cover losses in purchasing power often go unreported and recent military expenditure can appear to be falling in real terms when it is in fact increasing. Unless exceptional uncertainty is involved in these estimates, they are not bracketed. The exceptions are Canada, the UK and the USA, for which NATO statistics report data on a fiscal-year basis. Calendar-year data are calculated on the assumption of an even rate of expenditure throughout the fiscal year. The exceptions are countries in transition whose economies are still so closed that MERs, which are based on price ratios in foreign transactions only, do not accurately reflect the price ratios of the entire economy.