What a stock index actually measures
The answer
An index is a weighted average of a chosen list of companies. It is not "the market". Which companies are in it, and how much each one counts, are decisions made by a committee — and in most large indices a handful of companies decide most of the movement.
Why this costs you money
"The market is up 1.2% today."
That sentence sounds like a fact about thousands of companies. Very often it is a fact about 5 of them.
In a market-cap-weighted index, a company's influence is proportional to its size. The largest members move the number; the smallest ones are close to decoration. When the top 10 names are more than half the index, the index is mostly telling you what those 10 did.
Two specific losses follow.
The first: you conclude your portfolio is doing badly when it is doing normally. The index rose 12% last year. Your 15 mid-sized companies rose 4%. You feel you have failed and you change strategy. But the index's 12% may have come almost entirely from its largest members, while the median stock in the market did roughly what yours did. You are comparing yourself against a number that was never a description of the average company.
The second, and worse: you think you are diversified when you are concentrated. An index fund feels like owning everything. In practice, a NIFTY 50 fund is a large bet on Indian financial services, and an S&P 500 fund has become a large bet on American technology. Both look like broad market exposure and neither is.
How it works
Three decisions define any index, and every one of them is a human choice.
1. Which companies. A committee selects them, usually by size and how easily they trade. The NIFTY 50 has 50 members. The Sensex has 30. The S&P 500 has about 500, chosen by a committee that also considers profitability.
2. How they are weighted. This is where most of the misunderstanding lives.
| Method | How influence is decided | Used by |
|---|---|---|
| Free-float market cap | By company size, counting only shares that actually trade | NIFTY 50, Sensex, S&P 500 |
| Price weighted | By the share price alone | Dow Jones Industrial Average |
| Equal weight | Every member counts the same | Equal-weight index variants |
Look at the middle row again. In a price-weighted index, a company whose shares cost $400 has 4 times the influence of one whose shares cost $100 — regardless of which company is bigger. If you read the first article in this cluster, you know the share price is an arbitrary number. The Dow, the most quoted index in the world, is built on it.
3. When the list changes. Indices are reviewed periodically and members are replaced. This has a consequence almost nobody states plainly: a company is usually added after it has already risen and removed after it has already fallen. The index buys high and sells low, mechanically, by design.
That sounds like a flaw. It is actually the engine. The index sheds its failures and keeps its winners, so it survives things that individual portfolios do not. Of the companies in the Sensex when it began, very few are still in it. The index's long record is partly a record of replacement.
What it tells you, and what it does not
An index tells you what a specific weighted basket did. That is genuinely useful — it is comparable over time, it is calculated the same way every day, and it gives you a benchmark.
It does not tell you what the average company did. For that you need the advance-decline figures — how many stocks rose against how many fell — or an equal-weighted version of the same index.
It does not tell you what a typical investor experienced, because typical investors do not hold the index in index proportions.
And it does not tell you the composition is stable. The S&P 500 of 2005 and the S&P 500 of today are the same name and a substantially different portfolio. Any comparison across decades is a comparison between 2 different baskets wearing one label.
The decision rule
Before you use an index as "the market", check 2 things.
- The top 10 weight. If the top 10 members are more than half the index,
you are watching those 10, not the market.
- The sector concentration. One sector above 30% means your "diversified"
index fund is a sector bet.
And before you judge your own performance against an index, ask whether the index rose broadly or narrowly. Cap-weighted rising far more than equal-weighted means a few giants carried it — and your portfolio was never in that race.
Try this now
Two minutes, and it changes what the word "market" means to you.
- Search for the NIFTY 50 factsheet on the NSE Indices website, or the S&P 500 factsheet, whichever matters to you. Both publish the top 10 constituents with weights, free, every month.
- Add up the weights of the top 10. Write down the total.
- On the same sheet, find the sector breakdown. Write down the largest sector's percentage.
- Now open your own portfolio. Work out what percentage your top 3 holdings are of your total, and what your largest sector is.
- Compare the 2 sets of numbers.
What you should see. In the NIFTY 50, the top 10 companies are a large majority of the index's movement, and financial services is by far the biggest sector. In the S&P 500, the top 10 have grown to a strikingly large share and technology dominates.
Then the uncomfortable half. Many investors who consider themselves diversified discover their top 3 holdings are a smaller share of their portfolio than the index's top 3 are of the index. You may be more diversified than the benchmark you are measuring yourself against.
Three real cases
1. The largest US technology companies, 2023-2024 — a few names, most of the gain A small group of very large technology companies accounted for a disproportionate share of the S&P 500's rise over this period. An investor holding a broad basket of the other 490 or so companies could have had a perfectly reasonable year while feeling they had badly lagged "the market". The index was not wrong. It was measuring something other than the average company.
2. General Electric leaves the Dow, June 2018 — the index replaces its dead GE was removed from the Dow Jones Industrial Average after more than a century as a member — the last of the original-era constituents. Its removal followed a long collapse in the share price. The index did not suffer GE's subsequent years, because GE was no longer in it. Every long-term index chart contains this survivorship, quietly.
3. Financial services in the NIFTY 50 (India) — the sector bet nobody makes consciously Banks and financial companies have long been the largest sector weight in India's headline index, by a wide margin. An Indian investor putting money into a NIFTY 50 index fund every month is, without ever deciding to, running a large and persistent position in Indian financial services. That may be a fine thing to own. It is not the same as owning the Indian economy.
The question that resolves it
A novice asks: how did the market do today?
An expert asks: how many stocks went up?
Those 2 questions have different answers surprisingly often, and the gap between them is where most of the confusion about "the market" comes from.
What would make this wrong
If index concentration did not matter, then a cap-weighted index and an equal-weighted version of the same list would move together. They diverge, often by a lot, and the gap is a direct measure of how narrow a rise has been.
The limits, honestly.
Concentration is not automatically a defect. Cap weighting concentrates in companies because they have grown, which is a reasonable thing for an index to do, and it is why index funds have beaten most active managers over long periods. An equal-weighted index is not the "true" market either — it just makes a different, equally arbitrary choice, and it needs constant rebalancing to maintain it.
And nothing here is an argument against index funds. For most people, in most decades, a low-cost index fund is a better decision than the alternatives they would otherwise make. The argument is narrower: know what is inside the one you own, and stop treating its number as a description of the average company.
In India
The NIFTY 50 and the SENSEX are the headline indices — 50 and 30 companies respectively, both weighted by free-float market capitalisation. Free-float means only the shares that actually trade are counted, so a company where promoters hold 75% gets a much smaller weight than its total size suggests. In a promoter-dominated market, this matters more in India than in most places.
Beyond the headline names:
- NIFTY Next 50, NIFTY Midcap 150, NIFTY Smallcap 250 and NIFTY 500 cover progressively more of the market. The NIFTY 500 is much closer to "the Indian market" than the NIFTY 50 is.
- Sector indices — Bank NIFTY, NIFTY IT, NIFTY Pharma and others — are where most Indian derivatives volume actually sits.
- Index reviews happen periodically, and the additions and removals are announced in advance. Index funds must then trade those names, which creates a known, dated flow.
The concentration point is sharper in India than elsewhere. India's headline index has both a high top-10 weight and a very high single-sector weight, so 2 different kinds of concentration stack on top of each other.
In the United States
Three indices are quoted constantly and they are built differently.
- The S&P 500 — around 500 large companies, free-float cap weighted, chosen by a committee that also applies profitability criteria. This is the serious benchmark.
- The Dow Jones Industrial Average — 30 companies, price weighted. It is the most famous index in the world and, by construction, one of the least meaningful. A share split changes a company's influence on the Dow without changing the company at all.
- The Nasdaq Composite — thousands of companies listed on the Nasdaq, heavily weighted towards technology.
The S&P 500 Equal Weight index exists and is published alongside the standard version. Comparing the 2 over any period is the fastest way to see whether a rise was broad or narrow, and it takes one chart.
Where they differ, and what that tells you
Both headline indices are concentrated. They are concentrated in opposite things.
The NIFTY 50 is dominated by financial services. The S&P 500 is dominated by technology.
What that tells you is that "global diversification" through 2 index funds may be less diversified than it looks — or more, depending on which risks you care about. An investor holding both is not holding "the world". They are holding Indian banks and American technology, plus a tail.
It also tells you the 2 indices respond to completely different things. An Indian rate decision moves the NIFTY through its banks. A shift in expectations about artificial intelligence spending moves the S&P through a handful of its largest members. Neither index is a thermometer for its country's economy, and treating one as a proxy for the other's drivers will mislead you every time.
Carry this
- An index is a weighted average of a chosen list. Both words are decisions.
- Check the top 10 weight and the largest sector before calling it "the market".
- Cap-weighted rising far more than equal-weighted means a few giants carried it.
- The index sheds its failures. Your portfolio does not do that by itself.