Arms Index [TRIN] Explained & The Mathematical Oddities You Need To Be Aware Of

Disclosure: Your support helps keep running! We earn a referral fee for some brokers & services we list on this page. Learn more...

The Arms Index , also know as the TRIN – “TRading INdex”, (ticker: $TRIN) is a volume-based confirmation indicator as well as overbought and oversold indicator.

The Arms Index has four components listed below:

  1. Advancing Issues on the New York Stock Exchange (NYSE) – $ADV or $NYADV
  2. Advancing Volume on the NYSE – $UVOL or $NYUPV
  3. Declining Issues on the NYSE – $DECL or $NYDEC
  4. Declining Volume on the NYSE – $DVOL or $NYDNV

The formula for the Arms Index is simply:

(Advancing Issues / Declining Issues) / (Advancing Volume / Declining Volume)

The intra-day 5-minute chart of the mini-Dow futures contract shows the $TRIN:

ADX technical analysis indicator

Interpreting the Arms Index

  • Neutral Reading = 1
  • Bearish Reading > 1
  • Bullish Reading < 1

The trend of the Arms Index is usually more important than whether or not the Arms Index is above or below 1. As can be seen in the intra-day chart above, when the mini-Dow was falling in price, the Arms Index was increasing.

At 1.5, a very high Arms Index reading, a trader might take a contrarian stance and buy at the 1.5 level. In addition a trader might consider waiting to see a reverse or bottoming of the Arms Index before taking such action. Also notice that when the mini-Dow is increasing, the Arms Index is decreasing.

The Arms Index can be used from a longer term perspective. Some traders use moving averages of the inputs into the Arms Index equation.

To illustrate: (10-day Moving Average (MA) of Advancing Issues / 10-day MA of Declining Issues) / (10-day MA of Advancing Volume / 10-day MA of Declining Volume) or one could simply take the 10-day Moving Average of the $TRIN.

There are fundamental problems with the Arms Index, and these probems are discussed on the next page.

Problems with the Arms Index

There are mathematical oddities with the Arms Index. To begin, the basic formula of the Arms Index is presented below:

(Advancing Issues / Declining Issues) / (Advancing Volume / Declining Volume)

According to the status quo interpretation of the Arms Index, a reading below one is bullish and a reading above one is bearish. However, what if there was a truly bullish day.

For this example, twice as many stocks will be gainers on the day than will be losers. Likewise, twice as much volume was transacted with the stocks that were gainers than was transacted with the losers of the day.

To recap, a very bullish day, gainers (Advancing Issues) outpace loser by a 2 to 1 ratio, and institutions are heavily accumulating stock of those gainers by a ratio of 2 to 1, basically lots of Advancing Volume. The calculation would be as follows:

(2/1)/(2/1) = 1.0

An Arms Index reading of one is supposed to be neutral, yet the example up above was extremely bullish. Suppose Advancing Issues were 3 to 1 and Advancing Volume was 2 to 1. This is yet another bullish scenario. This calculation is shown next:

(3/1)/(2/1) = 1.5

A reading of 1.5 is extremely bearish, yet Advancing Issues outpaced Declining Issues by 3 to 1 and Advancing Volume outpaced Declining Volume by a ratio of 2 to 1; very bullish inputs.

By reversing the previous scenario and making Advancing Issues stronger by a ratio of 2 to 1 and Advancing Volume stronger by a ratio of 3 to 1, the status quo interpretation of the Arms Index is found.

(2/1)/(3/1) = 0.67

The result, 0.67, is a strong bullish reading confirming this calculation's bullish inputs of 2 to 1 Advancing Issues and 3 to 1 Advancing Volume.

What Does This All Mean?

The Arms Index emphasizes volume. In the first example, there was twice as many Advancing Issues and twice as much Advancing Volume.

Therefore, 67% (2 to 1 is equal to a 67% and 33% split) of all NYSE issues were advancing, and subsequently 67% of all NYSE volume was with Advancing Issues.

For the Arms Index to give a bullish reading, the Arms Index would expect there to be more volume poured into the advancing stocks, such as in the last example where 75% (3 to 1 is 75%) of all NYSE volume was in Advancing Issues.

Nevertheless, a person could legitimately question why 67% of all NYSE issues being gainers and 67% of all volume being with winning issues could be considered neutral.

Arms Index Alternatives

Separating the two components of the Arms Index is a viable alternative.

  • (Advancing Issues / Declining Issues) is called the Advance/Decline Ratio
  • (Advancing Volume / Declining Volume) is called the Upside/Downside Ratio.

Another alternative to look into is creating an average volume per issue; this is shown below:

  • (Advancing Volume / Advancing Issues) / (Declining Volume / Declining Issues)

This version is more intuitive; it would be bullish above one because there would be more average volume per up issue and the indicator would be bearish below one because there would be higher average down volume per issue.

The same type of problems could arise as those with the Arms Index. In this case, very few issues could be gainers, but the stocks that were gainers could have high amounts of volume.

Thus the top half of the equation would be greater than the bottom half, returning a bullish signal, even though very few stocks advanced that day.

Probably the best alternative to the Arms Index is the market thrust indicator (see: Market Thrust); its formula is as follows:

  • (Advancing Issues x Advancing Volume) – (Declining Issues x Declining Volume)

With the market thrust indicator, the more advancing issues, the larger the market thrust value. Similarly, the more advancing volume, the larger the market thrust value.

Of course, by using math and an imagination, a trader could come up with various other combinations.

Moral of the story: It is important to be a knowledgeable trader and know what the strengths and weaknesses of each technical indicator is; also, don't rely on just one indicator.

How to Start Technical Trading

If you are interested in trading stocks & commodities using technical analysis, have a look at our reviews of these regulated brokers available in to learn which charting & analysis tools they offer:

Loading table...

CFDs are complex instruments and come with a high risk of losing money rapidly due to leverage. Between 73.0%-89.0% of retail investor accounts lose money when trading CFDs. You should consider whether you understand how CFDs work and whether you can afford to take the high risk of losing your money.

Further Reading

Learn more about technical analysis indicators, concepts, and strategies including Moving Averages, Candlestick basics, Gaps (windows), MACD, and many others.

Also see our guides on Forex, Bitcoin, CFD, and Option brokers to find out which technical charting tools online brokerages offer their clients.


[[{{Country}} Welcome]]
[[{{Country}} Welcome]]