I received an excellent comment and question in the mailbag, so I thought I’d turn it into a Rude School piece. Asking simple questions does one great credit; there’s plenty of room for misinterpretation, misreading, and misunderstanding. So, I’ll get the equity indexes hammered out for you today.

Here was the question:

Of all the PP articles I read each day, yours and Dave G’s 5-bullets are virtual “can’t miss” content. Thank you for sharing your insights and doing so in a way that financial schlubs like me have a chance to learn. One question has plagued me for years, and I think it may be a perfectly tidy topic for you to address and your readers to enjoy… What is the difference between the Dow, the NASDAQ, and the S&P 500, and why should I watch any one or all of them for market trend insights? Sincere thanks, once again.

I genuinely hope you enjoy a peaceful holiday weekend,

Andrew R. 

Thanks for the kind words, Andrew. I know Dave would love to know you read his work daily. I try never to miss Dave’s Bullets, either!

But I must stress something: no man is a schlub if he’s trying to learn. It takes a lot to admit to not knowing something and then set out to fill that knowledge gap. So, it’s my pleasure to clarify things for you here.

Let’s get to it.

What’s an Index?

It’s funny how I stress the need for a global perspective nowadays. But really, the only place you need to trade or invest in is the U.S. stock market.

But what of these indexes? What do they tell us? How are they calculated?

First, let’s define what an index is.

When Charles Dow first calculated the Dow Jones Industrial Average, he was looking for the general market direction. He chose what he thought were the 30 most important stocks in the U.S., added their prices together, and divided by 30. The result was, quite literally, an average of the U.S. industrial stocks.

The DJIA was the first formulation of a Western stock index. Let’s continue with it:

The Dow Jones Industrial Average

We still calculate the Dow the same way today. This is what we call a price-weighted index.

Price-Weighted Index Value = (∑ Pᵢ) / D

Where:

• Pᵢ = Price of the ith stock in the index (adding up the 30 equities’ stock prices)
• D = Divisor (adjusted for stock splits, dividends, and index component changes)

The divisor ensures the index value remains consistent despite adjustments to the index's composition. If Stock A splits, an unadjusted index will fall for no economic reason; hence, the divisor adjustment. As of November 2024, the Dow Divisor is 0.16268413125742, a far cry from 30.

A price-weighted index is biased toward higher-priced stocks, regardless of the company's actual market value. This is why institutional investors shy away from these kinds of indexes. (To visualize this, imagine a stadium filled with everyday sports fans. Then, imagine Warren Buffett comes in and sits in a seat. The average wealth per attendee would suddenly be much higher because of Buffett’s attendance.)

Also, since The Wall Street Journal’s editorial board still picks the Dow stocks, the index makeup can be construed as entirely arbitrary. (The “Dow” in “Dow Jones” was the same Charlie Dow.)

Where the Dow comes in handy is using it with Dow Theory.

Dow Theory is a framework for analyzing market trends that our friend Charles Dow invented in the late 1800s. It focuses on identifying and confirming the direction of major market trends. A key tenet of Dow Theory involves the relationship between the Dow Jones Industrial Average (DJIA) and the Dow Jones Transportation Average (DJTA).

The DJIA and the DJTA in Dow Theory

The relationship between these two indexes is crucial in Dow Theory because they represent different but interrelated aspects of the economy.

The DJIA reflects the performance of large, established companies across various sectors, representing the production side of the economy.

The DJTA tracks transportation and logistics companies (railroads, shipping, and air freight), representing the movement of goods.

A strong economy should show alignment between production and the ability to move goods to market.

We get a bullish confirmation if the DJIA hits new highs and the DJTA follows with new highs, because it suggests economic strength and validates a bullish trend.

We get a bearish confirmation if the DJIA and the DJTA hit new lows. It confirms economic weakness and a bearish trend.

If one index rises while the other falls (for example, if the DJIA makes new highs, but the DJTA lags or declines), we have a divergence. This warns us of a trend reversal.

Let’s move on to the S&P 500.

The SPX

The S&P 500 (SPX) tracks the performance of 500 large-cap U.S. companies across all sectors, providing a broad representation of the U.S. stock market. Like the NASDAQ, it is market capitalization-weighted.

A market capitalization-weighted index assigns weights based on the total market value of the component companies.

Market Cap-Weighted Index Value = ∑Pᵢ × Qᵢ ) / D

Where:

Pᵢ: Price of the ith stock.

Qᵢ: Number of shares outstanding for the ith stock.

Pᵢ × Qᵢ: Market capitalization of the ith stock.

D = Divisor: An arbitrary value set to scale the index to a desired starting level and adjusted over time for corporate actions or changes in the index composition.

Larger companies (with higher market caps) influence the index movement more than higher-priced stocks. The divisor is adjusted similarly to the Dow’s divisor.

The SPX is a diverse snapshot of the overall market. Because of its broad coverage, it’s considered the best gauge of the U.S. stock market. Fund managers, institutional investors, and even retail investors use it as a benchmark to evaluate their performance versus the market’s. (The difference is known as alpha.)

The NASDAQ Composite

The Nasdaq tracks over 3,000 companies listed on its exchange. Like the SPX, it’s heavily weighted toward technology and growth stocks (for example, Microsoft, Tesla, Amazon) and market capitalization-weighted.

It focuses on technology, biotech, and high-growth sectors. These stocks are often more volatile but can drive overall market innovation.

It’s also helpful in understanding market sentiment around riskier, high-growth investments.

Wrap Up

Using these indices together helps paint a picture of market performance and sentiment. The Dow highlights stability in well-established companies. The NASDAQ reflects the performance of growth-oriented, innovative industries. The S&P 500 gives an overall market view and is considered the best indicator of economic health.

Watching all three can provide insights into different market segments. For example, a tech rally will first appear on the Nasdaq. Broader economic trends will manifest in the S&P 500, and the Dow reflects confidence in blue-chip stocks.

Monitoring these indices helps identify trends, such as shifts from risk-on (growth stocks) to risk-off (value stocks) investing.

I hope that helps, Andrew.

Have a wonderful weekend!