How a Stock Index Actually Works: S&P 500 vs. Nasdaq vs. Dow

What an index is (and isn't)

A stock index is a math formula, not a fund. It is a rules-based list of companies plus a recipe for how much each company counts. The index itself owns nothing — it just publishes a number.

Every index answers three questions in its rulebook (the "methodology"):

• Selection: Which companies get in? (Size minimums, profitability tests, exchange listing, sector rules, or simply a committee's judgment.)
• Weighting: How much does each company count? This is the single biggest driver of how an index behaves — more than which stocks are in it.
• Maintenance: When are members swapped and weights reset? This is rebalancing and reconstitution.

When you buy an "index fund" or ETF, you're buying a fund that promises to copy that formula. The fund does the actual buying and selling; the index just tells it what the target looks like.

The big three, compared

Why the same market day looks different in each index

Because the weighting math differs, the three indexes can tell different stories about the same day. If mega-cap tech rallies, the Nasdaq-100 jumps hardest (most concentrated in tech), the S&P 500 follows (tech is its largest sector but not the whole index), and the Dow may barely move — unless one of its high-priced stocks happens to be involved. A 5% move in the Dow's highest-priced stock moves the index far more than a 5% move in a much larger company that happens to have a low share price. That's the price-weighting distortion in action.

Who decides: committees, rules & governance

There are two governance models in indexing. Committee-driven indexes (S&P 500, Dow) publish criteria but leave the final call to a group of people. Rules-driven indexes (Nasdaq-100, most Russell and total-market indexes) run on a formula — if you meet the rules, you're in, no vote required.

The S&P 500: a committee with published guardrails

The S&P 500 is managed by the U.S. Index Committee at S&P Dow Jones Indices — full-time employees of the index provider who meet monthly. Their deliberations are confidential, and they follow a published methodology, but the methodology explicitly gives them discretion: meeting every criterion makes a company eligible, not guaranteed entry. The committee also considers sector balance, so the index roughly mirrors the sector mix of the eligible large-cap universe.

To be eligible for addition, a company generally must have:

• U.S. domicile and a listing on an eligible U.S. exchange (NYSE, Nasdaq, Cboe)
• Market cap above the committee's threshold — a figure the committee raises periodically as the market grows (it has been in the ~$18–20B+ range in recent years)
• Positive earnings: GAAP profit in the most recent quarter and summed over the trailing four quarters — this is the test that kept Tesla out until late 2020 despite its enormous market cap
• Sufficient public float and liquidity — enough shares in public hands and enough trading volume for index funds to actually buy the stock without distorting it
• Appropriate security type — common equity; no ETFs, closed-end funds, LPs, or most foreign issuers

How a stock gets added

How a stock gets removed

Deletions come in two flavors. Involuntary/mechanical removals happen immediately when the company effectively ceases to exist as an independent public stock: it's acquired, merges, goes private, files for bankruptcy, or is delisted by the exchange. Discretionary removals happen when the committee judges a shrinking company no longer represents the large-cap market — typically after its market cap has fallen far below the entry threshold for a sustained period. These are often bundled into the quarterly rebalancing dates, and the removed company usually moves "down" into the S&P MidCap 400 or SmallCap 600 rather than vanishing entirely.

The Nasdaq-100: mostly formula, little discretion

The Nasdaq-100 is closer to pure rules. To qualify, a company must be listed on the Nasdaq exchange, be non-financial, meet a minimum average daily trading volume, and have traded for a seasoning period after its IPO. Once a year, in December, Nasdaq re-ranks all eligible companies by market capitalization: the top companies stay or enter, and members whose rank has fallen well below the top 100 are dropped (a buffer prevents companies from bouncing in and out on small ranking changes). Between annual reviews, a member is replaced immediately if it's acquired, delisted, or transfers its listing — and the methodology permits special rebalances when concentration limits are breached, as happened in July 2023. Note there's no profitability test at all, which is why fast-growing, not-yet-profitable companies can enter the Nasdaq-100 years before they'd qualify for the S&P 500.

The Dow: pure judgment

The Dow is chosen by the Averages Committee, which includes representatives of S&P Dow Jones Indices and The Wall Street Journal — a legacy of the index's newspaper origins. There are no quantitative criteria. The committee looks for companies with an excellent reputation, sustained growth, and broad investor interest, while keeping the 30 names roughly representative of the U.S. economy. Because the index is price-weighted, the share price itself is a selection factor: a stock priced in the thousands would dominate the index, so companies often become viable Dow candidates only after a stock split. Nvidia's addition in November 2024 (replacing Intel) followed its 10-for-1 split for exactly this reason. Changes are rare — often years apart — and are frequently triggered by an event elsewhere, such as a member being acquired or a big split reshuffling the weights.

Oversight: who watches the index makers?

Index providers publish their methodology documents publicly, maintain internal separation between the teams that make index decisions and the commercial side of the business, and follow announcement policies designed to give all market participants the news at the same time. In the EU, benchmark administrators are formally regulated under the Benchmark Regulation (BMR); U.S. oversight is lighter and relies more on disclosure and the providers' own governance frameworks. The practical concern for investors is the "index effect": because trillions of dollars must trade on inclusion news, added stocks have historically popped and deleted stocks dropped between announcement and effective date — though this effect has weakened over time as arbitrageurs anticipate changes.

Weighting: where index behavior really comes from

Take the exact same five companies and weight them three different ways — you get three different portfolios with different risk, different winners, and different rebalancing needs.

Market-cap weighting (and the "float-adjusted" detail)

Weight = company's market value ÷ total value of all members. Most major indexes use the float-adjusted version: shares locked up by insiders, founders, or governments are excluded, so the weight reflects what public investors can actually buy. Cap weighting is self-maintaining — when a stock rises, its price and its weight rise together automatically, so the index fund doesn't need to trade. That's why cap-weighted funds have the lowest turnover and lowest costs. The trade-off: winners keep growing their share, so the index concentrates into whatever has already gone up.

Price weighting

Weight = share price ÷ sum of all share prices. Only the Dow (and Japan's Nikkei 225) still work this way. It has no economic logic — a stock split cuts a company's weight in half overnight without changing its value. It survives because the Dow's 130-year history is itself the product.

Equal weighting

Every member gets an identical slice (0.2% each in a 500-stock index). This dramatically boosts exposure to the smaller members and dilutes the mega-caps — an equal-weight S&P 500 behaves more like a mid-cap fund. Because prices drift every day, equal weight requires regular rebalancing back to target, which forces a "trim winners, buy laggards" discipline — and creates higher turnover, trading costs, and (in taxable accounts) more taxable events. The best-known vehicle is RSP, the Invesco S&P 500 Equal Weight ETF, which resets quarterly.

Revenue weighting (and other fundamental weights)

Weight = company's revenue ÷ total revenue of all members (RWL, the Invesco S&P 500 Revenue ETF, is the main example — same 500 companies as the S&P, re-weighted by sales). The idea: tie the weight to the size of the underlying business instead of the market's opinion of the business. High-revenue, lower-margin companies (retailers, healthcare distributors, energy) get bigger weights; richly valued, high-margin software companies get smaller ones. In practice this produces a value tilt. Cousins in the same family weight by earnings, dividends, book value, or a blend.

How rebalancing works

Two different maintenance jobs get lumped under one word. Reconstitution changes who's in the index. Rebalancing resets how much each member counts. Index funds then trade to match, usually at the closing price of the effective date.

Each index on its own clock

• S&P 500: weights are refreshed quarterly (March, June, September, December) to update share counts and floats. But membership changes happen whenever needed — a merger, delisting, or bankruptcy triggers an immediate replacement, and the committee adds/drops companies year-round.
• Nasdaq-100: full reconstitution once a year in December (who's in the top 100), plus quarterly weight rebalances. Its methodology also allows special rebalances: in July 2023, mega-cap tech had grown so dominant that Nasdaq trimmed the top weights outside the normal schedule to satisfy concentration limits.
• Dow: no calendar at all. The committee swaps a member when it decides one company no longer represents the economy well. When a swap or stock split happens, the divisor is adjusted so the index level doesn't jump artificially.
• Equal-weight & revenue-weight funds: rebalancing is the whole engine. RSP resets every stock back to equal weight quarterly; revenue-weighted indexes re-weight as new annual sales figures arrive. Without the reset, they'd drift back toward cap weighting.

How to choose: size, revenue, or equal weight?

There is no universally "best" weighting — each one is a bet on something. The honest way to choose is to know which bet you're making.

A practical decision guide

Checklist before buying any index fund

• Read the weighting method first — it predicts behavior better than the fund's name.
• Check the top-10 weight — how much of the fund is really just a few stocks?
• Compare expense ratios — cap-weight funds run as low as 0.03%; alternative-weight funds typically cost 0.2%–0.4%.
• Check turnover — high turnover means hidden trading costs and, in taxable accounts, more distributions.
• Know the rebalance schedule — it tells you how quickly the fund adapts (or doesn't) to a changing market.