What Is an Index?

The instruction manual, finally opened

In Chapter 1's picture, the ETF is the boxed set and the index is the instruction manual. This chapter opens the manual.

Every serious index document, usually a public PDF called a methodology, answers five questions.

1. The universe. Which securities are even eligible? (US-listed common stocks; developed-market large caps; euro-denominated investment-grade bonds.)
2. The selection rule. From that universe, which make the list? (The 500 largest; everything above a size cutoff; a committee's judgment within criteria.)
3. The weighting rule. How much of each? This is the big one, covered below.
4. The maintenance schedule. When is the list refreshed (quarterly, semi-annually), and what happens between refreshes when a company is acquired or collapses?
5. The edge-case handbook. Caps on any single holding, buffers to stop stocks bouncing in and out, rules for suspended stocks, dividend treatment.

Nothing in the manual says "we think Nvidia will do well." Indices don't have opinions; they have criteria. When a company stops meeting the criteria it leaves the list, and the ETF sells it. No meeting, no fear, no hope. That mechanical ruthlessness sounds like a limitation but is the feature: it is how the index quietly ejected its failures and admitted its future winners for a century, and it is what you are buying when you buy the tracker.

Weighting: the decision that defines your returns

Two indices can hold the same 500 companies and deliver meaningfully different results, because weighting decides where your money concentrates.

Market-cap weighting is the near-universal default (S&P 500, MSCI World, FTSE All-World). Each company's weight equals its share of the list's combined market value. If Apple is worth about 5% of all 500 companies' combined value, about 5% of your money is in Apple. Two properties make cap-weighting the champion.

• It is self-maintaining. Prices move, weights move with them, and the fund barely needs to trade; the index "rebalances itself." That is why cap-weighted trackers are the cheapest funds on earth.
• It reflects the market's actual verdict. You own the market as it is, not as any rulewriter wishes it were.

Its honest cost: concentration follows success. When a handful of giants dominate, as US mega-caps have in recent years with the top 10 stocks at times exceeding a third of the S&P 500, your "500-company" fund is more top-heavy than its name suggests. That is not a flaw but a property, and one you should know you own.

The main alternatives, previewed here and detailed in Chapter 45:

• Equal weighting. Every company gets the same slice (0.2% each in a 500-stock index). More small- and mid-cap exposure, a mechanical buy-low/sell-high at each rebalance, higher trading costs.
• Free-float adjustment. A refinement of cap-weighting used by all major providers: only shares actually available to trade count, so a founder's locked-up 40% stake doesn't.
• Factor and fundamental weighting. Weight by dividends, value metrics, volatility and so on. Rule-based, but expressing a view, and the moment a manual expresses a view you are choosing a strategy, not "the market."
• Price weighting. The fossil record: the Dow Jones Industrial Average weights by share price, so a $400 stock counts four times as much as a $100 one regardless of company size. It survives on fame; nobody would design it today.

Who writes the manuals, and the committee secret

Indices come from an industry of their own, dominated by three houses (MSCI, S&P Dow Jones Indices and FTSE Russell) plus challengers such as Solactive and Morningstar competing mainly on price. Fund providers pay licensing fees to track branded indices, a cost that flows into your TER, and the challenger indices are one reason some excellent ETFs are cheaper than their famous-index twins. Chapter 39 has more, including how the same "world" is sliced differently: MSCI counts South Korea as emerging, FTSE as developed, a genuinely useful pub fact that occasionally moves billions.

One myth worth killing early: not every index is a pure algorithm. The S&P 500 has a committee. Its selection criteria are published (US domicile, size, liquidity, positive earnings), but humans apply them with discretion, which is why "the 500 largest US companies" is almost but not exactly true. Tesla famously waited well past qualifying size. MSCI and FTSE run more mechanically. In practice the difference is small; philosophically, it is worth knowing that your "passive" fund may follow a refereed list, not a formula.

And a scale check, because the numbers are absurd: index providers maintain millions of indices, vastly more indices than there are stocks. Most are unused benchmarks, and only a few thousand have real money tracking them. "It tracks an index" is therefore, by itself, a nearly empty sentence. Which index is the entire question, a lesson that peaks in the thematic-ETF chapter (47), where indices are sometimes reverse-engineered to fit a product idea.

What an index can't do

• It can't hold cash or flinch. In 2008, in 2020, whenever markets fall, the index rides it down fully, and so does your tracker. The plan for that is asset allocation (Chapters 18 and 26), never the index itself.
• It isn't automatically diversified. A "Global Semiconductor Top 10" index is a rulebook too. Rules are not breadth; check both.
• It has costs it doesn't show. The index is frictionless math. The fund tracking it pays real-world trading costs, especially around rebalance days when everyone must trade the same names at once (the index-effect story of Chapter 41). The gap between manual and reality is called tracking difference: Chapter 35, one of the most useful chapters in this book.

A worked example: one rule, end to end

The FTSE 100, simplified to essentials: take the 100 largest companies listed on the London Stock Exchange, weight by free-float market cap, review quarterly, with a buffer (companies enter at rank 90 or above, exit at rank 111 or below, to stop borderline names churning).

Follow one event through the machine. A FTSE 100 constituent is acquired and delisted in March. At the review, the buffer rule promotes the largest FTSE 250 company into the vacancy. Every FTSE 100 tracker on earth sells the departed name and buys the promoted one, in index weight, on the same effective date: automatically, transparently, and predictably enough that Chapter 41 discusses who trades ahead of it. You, the investor, did nothing and missed nothing.