I used to think of a stock index the way I think of a thermometer — a neutral instrument reporting a fact about the world. That is not what an index is.
An index is a rulebook, applied by an organisation that sells the results, and the rules involve real choices.
Somebody decides what goes in
The most famous large-company index in the United States is not simply the largest companies. A committee selects constituents against published criteria, and it exercises judgement within them.
Criteria typically include listing location, a minimum market value, sufficient trading volume, a public float above some threshold, and a profitability requirement.
But meeting the criteria does not entitle a company to inclusion. The committee decides, and it has left qualifying companies out for extended periods.
Other index families are fully rules-based with no committee discretion. Both approaches exist and they produce different results.
Then somebody decides how much of each
Weighting is the second choice, and it matters more than most people realise.
Market capitalisation weighting gives each company a share proportional to its total value, which means the largest companies dominate. This is the most common approach.
Equal weighting gives every constituent the same share, which tilts the result toward smaller members and requires constant rebalancing.
Price weighting — used by one very old and very famous index — weights by share price alone, which is essentially arbitrary since share price reflects how many shares exist rather than company size.
Fundamental weighting uses revenue, book value or dividends instead of market price.
Each of these produces a different number from the same underlying companies. None is the true one.
Free float changes the arithmetic
Most modern indices weight by the shares actually available to trade rather than by all shares outstanding.
So a company where a founding family or a government holds a large block gets a smaller index weight than its headline value would suggest.
This makes sense — an index fund cannot buy shares that are not for sale — and it means index weight and company size diverge in ways that are not obvious from the outside.
Reconstitution is a scheduled event
Indices are reviewed on a calendar, with additions and deletions announced in advance and taking effect on a set date.
Because trillions of pounds track these indices passively, the funds must trade on that date to match the new composition.
Which creates a predictable, enormous, one-directional flow — and other market participants know about it and position ahead of it.
The effect on prices around reconstitution has been studied extensively and it is real, though it has diminished as index providers have made the process less predictable in its details.
Inclusion is not an endorsement
When a company is added to a major index, its share price frequently rises on the announcement.
This is sometimes reported as the market approving of the company. It is mostly mechanical — index funds must now buy it, and that demand moves the price.
Which means the price move tells you about fund flows rather than about the business, and reading it as a quality signal is a mistake.
The index provider is a business
Index providers license their indices to fund managers, who pay a fee based on assets tracking them.
Which means the provider has a commercial interest in its indices being widely tracked, and in creating new indices that fund managers want to launch products against.
This is not sinister and it is worth knowing, because the proliferation of narrow thematic indices reflects product demand as much as analytical insight.
What this means for a fund buyer
Two funds both described as tracking the same market can hold meaningfully different things, because they track different indices with different rules.
The fund document names the specific index. That name is worth looking up, and the index methodology document is public.
It is dry reading and it will tell you the selection criteria, the weighting scheme and the review schedule, which together determine what you actually own.
The honest summary
None of this argues against index investing, which remains a reasonable default for most people for reasons that have nothing to do with index construction being neutral.
It argues against treating an index number as a fact of nature. It is a construction, and knowing how it was constructed changes how much weight the number deserves.
I am not qualified to tell anybody what to hold, and this is a description of how these instruments are assembled rather than advice about using them. Anyone making decisions with real money should be talking to someone regulated to advise.
The float adjustment nobody notices
One more mechanical detail with visible consequences.
When a company's free float changes — a lock-up expires, a government sells down a stake, a founder distributes shares — the index weight changes even though nothing about the business has.
Which forces tracking funds to trade, and produces price pressure entirely unconnected to the company's prospects.
Index providers publish a calendar of these adjustments and the market anticipates them, which is why a lock-up expiry can move a price before any shares are actually sold.