The big indices: S&P 500, Nasdaq, Dow

THE S&P 500

The Standard & Poor's 500 is the broadest of the three indices in common use. It contains the 500 largest U.S. publicly-traded companies, selected by the S&P index committee based on a set of criteria (size, liquidity, profitability, U.S. domicile).

The S&P 500 is market-capitalization weighted. That means each company's influence on the index is proportional to its total market value. Apple, with a market cap around $3.5 trillion, has roughly 7% weight in the index. A smaller S&P 500 company with a $20 billion market cap has roughly 0.04% weight.

This weighting matters because it means the S&P 500's daily move is dominated by the largest companies. If Apple, Microsoft, Nvidia, and a few other megacaps all rise 1% while the rest of the index is flat, the S&P 500 will print a meaningful gain. If those same megacaps fall while the rest is flat, the index falls. The "average S&P 500 stock" can have a very different day from the index itself.

The S&P 500 represents roughly 80% of the total market capitalization of all U.S. publicly-traded companies. When financial media says "the market," they usually mean the S&P 500.

THE NASDAQ COMPOSITE

The Nasdaq Composite is broader than its name suggests. It contains roughly 3,000+ companies — every common stock listed on the Nasdaq exchange. Because Nasdaq attracted technology companies in the 1980s and 1990s, the Composite is heavily tech-weighted: Apple, Microsoft, Nvidia, Amazon, Meta, Alphabet, Tesla, and Broadcom collectively account for roughly 40% of the index.

This concentration is the central feature of the Nasdaq Composite. When tech is leading the market, the Nasdaq outpaces the S&P 500. When tech is lagging or correcting, the Nasdaq lags or falls more. A typical day where the S&P 500 is up 0.5% might see the Nasdaq up 0.9% (tech leading) or down 0.2% (tech rotating out). The gap between the two is one of the simplest signals for reading sector rotation.

There's also a more focused version called the Nasdaq 100, which contains the 100 largest non-financial companies on Nasdaq. The Nasdaq 100 is the index tracked by the popular QQQ ETF. It's even more tech-concentrated than the broader Composite.

THE DOW JONES INDUSTRIAL AVERAGE

The Dow is the oldest of the three, dating to 1896. It's also the most idiosyncratic. It contains only 30 large U.S. companies, hand-picked by the editors of the Wall Street Journal to represent a cross-section of American industry.

The Dow is price weighted, not market-cap weighted. This is unusual. In a price-weighted index, the influence of each stock is proportional to its share price, not its company size. UnitedHealth at $580 a share has roughly 8× the weight of Apple at $245 a share — even though Apple's market cap is several times larger. A 1% move in UnitedHealth moves the Dow more than a 1% move in Apple.

Price weighting has no theoretical justification — it's a historical artifact from 1896 when computing market caps continuously was impractical. Modern indices are market-cap weighted because that's what actually represents ownership in the underlying economy. The Dow persists because of brand and tradition, not because it's a useful measure.

For most analytical purposes, ignore the Dow and use the S&P 500 instead. The Dow is fine as a headline reference — "the Dow closed up 200 points" — but you should not be making any trading decisions based on it.

USING THE INDICES TOGETHER

The three indices, read together, tell you something the individual numbers don't.

S&P 500 up + Nasdaq up more: tech is leading. This is the most common configuration in growth-friendly markets, and it favors the kind of growth-stock setups CAN SLIM is built around.

S&P 500 up + Nasdaq up less: defensive sectors are leading (utilities, staples, healthcare). This is often a late-cycle signal — investors are still buying, but rotating to safer names. Worth watching if it persists for a couple of weeks.

S&P 500 flat + Nasdaq down: tech is rotating out while the broader market holds. This is a signal that growth-stock setups are getting harder, even if the headline market looks fine. CAN SLIM-style traders should reduce position sizes or sit out.

S&P 500 down + Nasdaq down more: full risk-off. Growth stocks fall faster than the broader market. We'll cover the formal counting rules ("distribution days," in O'Neil's vocabulary) in Month 7.

Dow up while S&P 500 and Nasdaq down: rare configuration; usually means a few large Dow components had idiosyncratic news (a single megacap earnings reaction) that's pulling the Dow against the broader tape. Generally ignore.

The point of comparing isn't to predict the next day's move. It's to read the character of today's tape. A 1% market move tells you very little; a 1% market move where tech outperformed financials by 2 percentage points tells you something specific about which kind of stocks are working right now.

ETFs THAT TRACK EACH INDEX

You'll see these tickers constantly:

• SPY — tracks the S&P 500. Largest ETF in the world by assets.
• QQQ — tracks the Nasdaq 100. Tech-heavy, growth-tilted.
• DIA — tracks the Dow. Less commonly traded.
• IWM — tracks the Russell 2000 (small-cap index, not covered today but worth noting). When small-caps are leading or lagging the S&P 500, that's another useful market-character signal.

For most BiST readers, you don't need to trade these ETFs. They're useful as reference points — when "the market" is up or down, you can pull up SPY, QQQ, and IWM and see at a glance which segment of the market is moving.