The Arms Index is calculated by first dividing the number of advancing stocks by the number of falling stocks: the Advance/Decline ratio. Then the volume of all advancing stocks is divided by the volume of all falling stocks: the Upside/Downside ratio. Then the Advance/Decline ratio is divided by the Upside Downside ratio.
The index is named after its developer, Richard W. Arms, Jr. It's also known as TRIN, which is short for TRading INdex.
The Arm Index can only be calculated for groups of stocks, not individual issues. It is not an indicattor like the MACD or RSI in that reguard. The symbol for the NYSE Arms Index is $TRIN. The symbol for the Nasdaq Arms Index is $TRINQ.
The Arms Index is usually considered a short-term trading tool, and is often combined with a moving average. The ARMS is used as an overbought/oversold indicator.
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