Understanding Arms Index

The Arms Index (also known as TRIN, an acronym for TRading INdex) was developed in 1967 by Richard Arms. It is a volume-based indicator that determines market strength and breadth by analyzing the relationship between advancing and declining issues and their respective volume. The Arms index is used to measure intraday market supply and demand, and it can be applied over short or long time periods, though it is primarily used as a short-term trading tool.

Calculation
The Arms Index is calculated by first dividing the number of stocks that advanced in price by the number of stocks that declined in price to determine the Advance/Decline Ratio. Next, the volume of advancing stocks is divided by the volume of declining stocks to determine the Upside/Downside Ratio. Finally, the Advance/Decline Ratio is divided by the Upside/Downside Ratio.

The Arms index is calculated as follows:

TRIN = (advancing issues/declining issues)
(volume of advancing issues/volume of declining issues)


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