1. What the S&P 500 really is and who decides who gets in

Level 11: The structure of the index

'The 500 largest companies in the United States.' That definition circulates in the press, in sales brochures and in after-dinner conversation, and it is inaccurate on all three counts: there are not exactly 500, size is not the decisive criterion, and nationality is not settled the way you would imagine. Starting here is not pedantry. If you are going to look for an edge around this index, the first thing is to know precisely what object you have in front of you.

An index is a portfolio with rules, not a photograph of the market

The S&P 500 in its current form starts in 1957, with a credit analysis house, Standard and Poor's, behind it. Today it is maintained by S&P Dow Jones Indices, and the composition decisions are taken by an index committee: a small group of people who meet regularly and whose identities are not published in detail. That committee does not vote on which companies it likes. It applies a written, public methodology. But it is worth seeing the whole play: that methodology is written by it, reviewed by it, changed by it, and it expressly reserves room to depart from it.

The consequence is uncomfortable for many people who index believing they are buying neutrality. The S&P 500 covers around 80% of US listed capitalization. That is an enormous portion of the market, but it is still a chosen portion, not the whole market. Buying the index is not 'buying the American economy': it is delegating to a committee which slice of it you hold.

The criteria: what you have to meet even to be eligible

The methodology demands several things at once, and failing one is enough to be left out:

• Domicile. The company has to be considered American on a set of indications: place of incorporation, headquarters, where it pays tax, where it primarily lists. It is not a single objective fact, it is a judgment.
• Minimum capitalization. There is a size threshold reviewed from time to time that has risen notably: a few years ago it was around eight billion dollars and today it sits far above that.
• Liquidity. A minimum volume traded in each of the preceding months is required, along with a minimum ratio between the money traded in a year and the available capitalization.
• Free float. A meaningful share of the stock has to be genuinely in circulation, not in the hands of founders, states or parent companies. The usual threshold is on the order of 10% of the capital.
• Recent profits. The sum of the last four quarters has to be positive and in addition the most recent quarter has to be positive on its own.

Pause on that last one. It is the most revealing, because it has nothing to do with size: a gigantic, liquid, American company is left out if it has just had a loss-making quarter. The most famous index in the world has an accounting profitability filter built in as standard. That is not a photograph of the market: it is a selection rule, and one the factor literature would recognize without hesitation.

Eligible does not mean in

Here is the point almost nobody internalizes: meeting all the criteria gives you eligibility, not a right of entry. The committee chooses among the eligible, and it can take its time, skip somebody or prioritize the index's sector balance over simple order of size. It can also remove a company that still meets the criteria, if it decides it no longer represents what the index is meant to represent.

The most discussed case of the last decade is Tesla's: it met the profits requirement in the summer of 2020, was not incorporated in the September review that year, and entered in December. Nothing changed at the company between one date and the other. What changed was a decision.

Large American companies that are not in it

A huge company not appearing in the index can be down to very different reasons:

• It is not listed. Enormous private companies are out by definition, and there are plenty of them.
• It lists in the United States but is domiciled abroad. Many companies with American business and operating headquarters are considered foreign for index purposes.
• It does not pass the profits filter. Companies in aggressive expansion that reinvest everything and carry accounting losses can be gigantic and remain ineligible for years.
• It is eligible and the committee has not called it yet. This happens constantly.

The practical conclusion: the index is not a ranking. It is a list curated by rules and by people.

What this implies for anyone seeking an edge

Three concrete implications, and none of them is advice about any security:

• If the index applies filters (profits, liquidity, float, domicile), then the index is already a strategy. The honest comparison for any system of your own is not against 'the market', it is against another strategy with explicit rules.
• If there is human judgment in the composition, there are decision dates, and decision dates are points where somebody is forced to trade. That will be module 4.
• If the index leaves out enormous chunks of American productive capital, then indexing does not eliminate the decisions: it transfers them to someone else. Which may be perfectly reasonable, but it is worth knowing.

Real case: when the committee changed the share class rule

In 2017, after Snap's flotation with non-voting shares for the public, S&P Dow Jones Indices announced that it would stop admitting companies with multiple share class structures into its main indices. Those already inside, such as Alphabet or Berkshire Hathaway, stayed. In 2023, the firm reversed that policy.

Note what happened and what did not. The economics of no company changed. Their size, their cash and their market share did not change. A rule changed, and with it the set of companies that could end up forming part of the portfolio millions of people buy. And one detail that closes the module with numbers: the S&P 500 usually contains around 503 listed lines, not 500, because a few companies have two classes of shares that trade separately within the index itself.

The name says 500. Reality says: whatever the committee has decided this week.