What Is the Arms Index (TRIN)? How to Use the Indicator

By Rebecca Lake · May 18, 2024 · 8 minute read

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What Is the Arms Index (TRIN)? How to Use the Indicator

The Arms Index or Trading Index (TRIN) is a breadth indicator used to indicate when the stock market is overbought and oversold. In simpler terms, it measures how strong or weak the market is on any given day.

Technical analyst Richard W. Arms developed the Arms Index formula in 1967 as a tool for gauging market sentiment. Investors still use TRIN indicators to track volatility and price movements. By looking for upward or downward trends in the TRIN and comparing them to other technical indicators, investors can potentially identify buy or sell signals.

Key Points

•   The Arms Index, also known as TRIN, measures stock market strength or weakness.

•   It was developed by Richard W. Arms in 1967 to gauge market sentiment.

•   TRIN calculates by dividing the Advance/Decline ratio by the Advance/Decline volume ratio.

•   A TRIN value above 1.0 suggests a bearish market, while below 1.0 indicates bullish conditions.

•   Investors use TRIN alongside other indicators to identify potential buy or sell signals.

What Is the Arms Index (TRIN)?

The Arms Index, Trading Index or TRIN for short is a breadth oscillator used to identify pricing and value trends in the stock market. Specifically, the index looks at two things: Advance Decline ratio and Advance Decline volume ratio.

The former represents the number of advancing and declining stocks while the latter represents advancing and declining stock volume.

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The TRIN Formula

TRIN = (Advancing stocks/Declining stocks) / (Composite volume of advancing stocks/Composite volume of declining stocks)

In this formula:

•   Advancing stocks refers to the number stocks trading higher

•   Declining stocks refers to the number of stocks trading lower

•   Advancing volume is the total volume of all advancing stocks

•   Declining volume is the total volume of all declining stocks

Investors use moving averages to smooth out the data and understand the relationship between the supply and demand for stocks during a given time period. The Arms Index aims to highlight bearish or bullish trends based on the relationship between the number of stocks being traded and the volume.

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How to Calculate TRIN

To calculate TRIN, you’d simply apply the formula outlined earlier. Again, it looks like this:

TRIN = (Advancing stocks/Declining stocks) / (Composite volume of advancing stocks/Composite volume of declining stocks)

Here’s what calculating TRIN might look like in action:

•   Find AD ratio by dividing the number of advancing stocks by the number of declining stocks

•   Find AD volume ratio by dividing total advancing volume by total declining volume

•   Divide AD ratio by AD volume ratio

You’d perform these calculations over a set time period, recording each figure on a graph or chart as you go. For example, you might space the calculations out every few minutes, hourly or daily. You’d then connect each data point on your graph or chart to whether the TRIN is moving up or down.

Dive Deeper: How to Calculate AD Ratio

What Does TRIN Show You?

TRIN shows you the market’s volatility and the short-term direction of prices to help investors identify buying opportunities. When reading or interpreting TRIN data, you’re looking to see if it’s above 1.0 or below 1.0. This can tell you whether the market is bearish or bullish. A reading of exactly 1.0 is considered neutral.

For example, a reading below 1.0 is common when there are strong upward trends in price movements. Meanwhile, a reading above 1.0 is typical when there’s a strong downward trend. Here’s another way to think of it. When the reading is below 1.0 that means advancing stocks are driving volume but when it’s above 1.0, declining stocks are in the driver’s seat for generating volume.

You may also look at the direction TRIN is moving. A rising TRIN could indicate a weak market, while a falling TRIN may mean the market is getting stronger. Understanding how to read the data matters when determining whether the market is overbought or oversold at any given time.

Overbought

In stock trading, overbought means a stock is selling at a price above its intrinsic value. When the market is overbought, there’s generally a bullish attitude as investors keep buying in and driving up market capitalizations.

But a sell-off can happen if market sentiment turns negative. In that case, you get a reversal and prices begin to drop, potentially pushing market capitalizations down. Investors use the Arms Index or TRIN to spot this type of price movement trend and get ahead of a reversal before it happens.

Oversold

When an asset is oversold it means it’s trading below its intrinsic value. In other words, it’s trading for less than what it’s actually worth. This scenario can happen if an asset is undervalued for an extended period of time.

When investors assume the market is oversold, that can lead to an increase in buying activity. This, in turn, can drive stock prices up.

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Example of Using TRIN

If you wanted to apply the TRIN in real time, you could do that using stock charts that illustrate technical indicators. So, say you want to track the movements of the S&P 500 Index for a single day, looking at prices in five-minute intervals. You begin calculating TRIN at 10:00 am, at which time it’s 1.10. This sends a sell signal to the market and prices begin edging down.

An hour later, you see that TRIN has dropped to 0.85 sending a buy signal. At this point, prices begin to move upward again. By following TRIN throughout the day you could see whether the upward trend looks like it will continue or whether it will eventually reverse. If you’re following the rule of “buy low, sell high“, you might want to time trades to correlate with stock price movements based on the trends forecasted by the TRIN.

How Is TRIN Different Than TICK?

The TRIN measures the spread or gap between supply and demand in the markets. The Tick Index or Tick Indicator shows the number of stocks trading on an uptick minus the number of stocks trading on a downtick. This trend indicator measures all of the stocks that trade on an exchange such as the New York Stock Exchange (NYSE) or Nasdaq.

Unlike Arms Index or TRIN, the Tick indicator does not factor in volumes. Instead, Tick index aims to pinpoint extreme buying or extreme selling on an intraday basis.

Is TRIN a Good Indicator?

The TRIN has both good points and bad points when used as an investment decision-making tool. No technical analysis indicator can yield precise answers when determining the best time to buy or sell.

It’s important to keep in mind that the Trading Index is just one indicator analysts use to evaluate the stock market and stock volatility. The TRIN is most helpful when used with other indicators in order to create a more comprehensive snapshot of the markets at a particular moment in time.

Pros of TRIN

The Arms Index or TRIN closely analyzes trends between advancing and declining assets. By comparing net advances to volume, it provides a picture of price movements. Volume can be a useful indicator in itself, as higher volumes can suggest more significant shifts in stock pricing.

The TRIN is forward-looking so it can be useful in forecasting which way the market will head next. By pointing out stocks that may be overbought or oversold, the indicator can provide investors with some direction when trying to buy the bottom or sell the top to maximize profits in the market.

Cons of TRIN

If the TRIN has one big flaw it’s that it may generate inaccurate readings because of the way the index accounts for volume. Specifically, you can run into problems when advancing volume falls short of expectations.

For example, say that on a given day the number of stocks advancing significantly outpaces the number of stocks declining. Meanwhile, the same trend happens with volumes, with advancing volume outstripping declining volume. When you calculate TRIN, the numbers could effectively cancel one another out, resulting in a neutral reading.

This can make it difficult to figure out if the market is trending bearish or bullish. For that reason, it may be helpful to apply a 10-day moving average (MA) to help even out the numbers and provide a more accurate picture of pricing trends.

How Investors Can Use TRIN

Technical investors can use the TRIN to analyze the market, decide whether to buy or sell, and when to make those trades to produce the best results. When using the index, you’re looking for clear markers of strength or weakness in the markets. By gauging overall market sentiment, it may become easier to make predictions about future prices.

The TRIN is, by nature, designed to monitor short-term trading activity so it may not work as well for spotting longer term trends. But you can use it to get a feel for whether the market is leaning more on the bullish or bearish side and how likely that trend is to either continue or reverse.

The Takeaway

The Arms Index or TRIN is an important concept to understand if you’re an active day trader using technical analysis. With technical analysis, you’re trying to find trends in the near term so that you can take action to capitalize them.

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Photo credit: iStock/Delmaine Donson


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