An index is a set of rules rather than a neutral measurement of a market.
Weighting methods
Market capitalisation, price and equal weighting.
Which produce very different indices from the same constituents.
Price weighting
Higher priced shares having more influence.
Which has no economic justification and persists in some famous indices.
Free float adjustment
Counting only shares available to trade.
Rebalancing
Periodic reconstitution generating trading.
Which index funds must follow.
Why the weighting choice matters
Market capitalisation weighting means the largest companies dominate, so an index of hundreds of companies can be driven by a handful of them.
Which is a real concentration risk hidden inside something described as diversified.
Equal weighting produces a different index entirely from identical constituents, with different returns, different volatility and higher trading costs.
Sector and regional concentration
Indices reflecting the composition of their market.
Which is not a neutral allocation.
Committee decisions
Some indices selected by judgement rather than rules.
Which is worth knowing about famous indices.
Index licensing
Providers earning fees from funds tracking them.
Choosing an index fund
Understanding what the underlying index actually holds.
What an index actually represents
A rules-based selection of securities, weighted by a chosen method, maintained by a commercial provider.
Which is a product rather than a measurement of the economy.
Two indices covering the same market can produce noticeably different returns because of construction choices that are documented and rarely examined.
Total return against price indices
Whether dividends are included.
Which makes a substantial difference over time and is frequently not stated in media coverage.
Smart beta
Indices weighted by factors other than size.
Which sits between passive and active.
Index methodology documents
Published by providers in full.
Which is where the rules actually are.
What to check
Constituents, weighting method and rebalancing frequency.
Why this matters when choosing a fund
Buying an index fund means accepting the index provider's rules about what to own and in what proportion.
Which is a delegated decision that most investors never examine.
Two funds both described as tracking a country's market can hold quite different things, and the methodology document explains exactly why.
Concentration in practice
Major indices where a handful of companies dominate.
Which has been the case in several markets recently.
Broad market indices
Covering more of the investable universe.
Which reduces concentration.
Index and benchmark selection
Funds measured against indices they choose.
Which affects reported performance.
A general note
This is description rather than a recommendation of any index or fund.
Why market structure is worth understanding
Most investment writing is about what to buy. Very little of it is about how the machinery underneath actually works: how orders are matched, how funds track what they claim to track, how prices respond to information, and what regulation does and does not cover.
That machinery determines a large part of the outcome. It sets what trading costs, it explains price movements that otherwise look irrational, and it defines the boundary between a risk you accepted and a failure someone else is responsible for.
None of it tells you what to invest in. It tells you what is happening when you do, which is a different and more durable kind of knowledge than any particular recommendation.
Where the reliable information is
Exchanges publish their rulebooks. Index providers publish their methodologies. Fund managers publish factsheets and annual reports. Regulators publish investor education material and firm registers. Companies publish annual reports and prospectuses.
All of it is free, all of it is authoritative, and almost none of it is promoted, because nobody makes money from you reading it. It is consistently more useful than commentary about it.
A general note
This article describes how markets and financial products work and is not investment, tax or financial advice. Rules, protections, tax treatment and market structure differ substantially between countries and change over time. Anything with money attached warrants checking against the rules applying where you are, and where a decision is significant, advice from a qualified professional who knows your circumstances is the appropriate route.
The recurring lesson
Across almost every topic here, the same pattern appears: a mechanism that looks arbitrary from outside turns out to follow published rules, and the rules are available to anyone who wants to read them.
The gap between what is knowable and what most investors know is not caused by secrecy. It is caused by the material being dry, unpromoted and available only to people who go looking for it, while a great deal of louder content competes for the same attention with considerably less substance.
Nobody needs to become an expert in market microstructure. Knowing that the answers exist, and roughly where, is enough to avoid the specific mistakes that come from assuming there is no answer.
Further reading
Regulator investor education material is written for the public, is free of any product to sell, and covers most of this ground more thoroughly than commercial content does.
It is the obvious place to start and almost nobody does.