The Arms Index, or TRIN, is an accurate measure of market trend and volume that can be a useful tool for identifying abnormal behavior and even spotting key reversal points in the markets.
Looking at breadth and depth of a market can give you early warnings of a new trend. By “breadth,” we simply mean volume. For any new trend to be sustained, there has to be at least steady, if not growing volume. Similarly, the majority of stock issues have to be moving in the same direction for the broader market to move in that direction. This is called “breadth.”
Having one or the other, quite frankly, isn’t really enough if you’re looking for the big trends. You really want to see both components in place for a market trend. You should be watching both market volume and the Advance/Decline (A/D) data. We’ll just reiterate that this information (especially volume) is one of the best tools a trader has when it comes to spotting a trend shift before it’s well underway.
But there is an easier way. You don’t have to gather four pieces of data, then plot moving averages of each one, and wait for crossovers for both sets. There is a single-line indicator that incorporates all the same data, and is just as useful and just as accurate.
It’s called the Arms Index (named after the developer, Richard Arms), but you may have heard it called by its more common name, TRIN. That’s short for "Trading Index."
Like the Advance/Decline and volume data, the TRIN indicator is exchange specific. For today’s illustration, we’ll be using the NYSE data to calculate our TRIN. You can also find TRIN readings for the Nasdaq exchange, but we’ve found that TRIN reading to be a little too volatile to use effectively. We’ll stick with the NYSE for right now.
Before we get into what TRIN is, just keep in mind the philosophy of how we apply this information; what we’re looking for here is balan! ce. If t he bullish volume is twice as strong as the bearish volume, that’s OK as long as the advancers are twice as numerous as the decliners. Everything would still be proportional.
If the advancers outnumber the decliners by a two-to-one ratio, but there is heavier volume on the selling side of the market, clearly things are out of whack—and that’s when reversals occur.
So now, let’s define TRIN and see how it’s used (and then we’ll look at why it’s particularly troubling right now).
As described above, the TRIN is a comparison of advancing and declining issues and the advancing volume and declining volume. It was designed by Richard Arms to highlight points in time when there is an imbalance in the advancers/decliners and the underlying trade volume of those advancers and decliners.
We’re using the Advance/Decline data for the New York Stock Exchange, but a TRIN reading can be calculated for any of the exchanges. The formula is as follows:
Arms Index (TRIN) = (# of Advancing Stocks / # of Declining Stocks) divided by (Volume of Advancing Stocks / Volume of Declining Stocks)
All things being equal, a perfectly balanced Arms Index would be 1.0. If the advancers or decliners become disproportionate with their corresponding volume, the Arms Index deviates from 1.0.
Of course, the market always finds its natural balance eventually, so when the TRIN levels are highly skewed, we know a big reversal is probably in the works.
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