The index reflects the trend in arms transfers abroad. Data are collected by the Stockholm International Peace Research Institute (SIPRI) from open sources. To measure the volume of international transfers, SIRPI has developed a unique system using a common unit, the Trend Indicator Value (TIV). The TIV is based on the known production costs of a major set of weapons.
SIPRI TIV data do not reflect the selling prices of arms transfers. Therefore, they should not be directly compared to gross domestic product (GDP), military expenditures, or the value of sales. The source advises using the information as raw data to calculate trends in international arms transfers over time.