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TRIN (Arms Index) The purpose of the TRIN (also called the ARMS Index after its inventor, Richard Arms) is to express the relationship of volume in advancing issues to volume in declining issues. When the market is rising we want to see more volume going into advancing issues than declining issues. When this doesn't happen, negative divergences occur. The opposite is true in declining markets. A chart of the index will display points where the market is oversold and overbought as well as divergences between the market and the TRIN. The basic raw calculation is as follows:
Advancing Stocks / Declining Stocks
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Advancing Volume / Declining Volume
This raw calculation can be used "as is"; however, most often the daily ratio is smoothed by moving averages of varying periods. Another way to calculate the TRIN is the "open" method, by which the ratio is calculated from a moving averages of the components rather than calculating a moving average of the daily ratio of the components. There are arguments for both methods, but I have settled on the open method and use a 10-day moving average because it is the most commonly followed. Here is the formula used for our charts:
Last 10 day's Advances / Last 10 day's Declines
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Last 10 day's Advancing Volume / Last 10 day's Declining Volume
Because of the numerous ways the TRIN may be calculated, be sure you know which method is being used. The most useful expression of the TRIN will be on a chart, and the more smoothing that is used the greater the time period the chart should display. This will allow you to compare current readings with prior ones and identify the normal range. On our charts we reverse the scale so that oversold readings show as bottoms and overbought readings as tops.