The Arms Index, also known as the TRading INdex (TRIN), is a technical analysis tool used by traders and investors to gauge the overall sentiment of the stock market. It was developed by Richard Arms in 1967.The Arms Index is calculated by dividing the number of advancing stocks by the number of declining stocks, known as the Advance-Decline Ratio (ADR), by the ratio of advancing volume to declining volume, known as the Volume-Advance Ratio (VAR).A value above 1.