What an Index Actually Measures

The question

"The news said 'the market is up today'. Up for whom? My father's shares went down."

Both statements can be true at the same time, and by the end of this chapter you will be able to explain exactly how.

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Home reading

One number, so you can compare two days

Ask "how did the Indian market do this year?" and you need a single number. Not 5,000 share prices — one.

That is all an index is: a single number that stands in for a group of companies, so you can compare today against last year. The Sensex does it with 30 companies, the Nifty with 50.

It is a summary. And like every summary, what it leaves out matters as much as what it keeps.

Why only 30, when India has crores of businesses?

Because an index is not trying to include everyone. It is trying to be a useful shorthand.

The companies are picked for a few reasons — they are large, their shares are traded heavily enough that the price means something, and together they cover different industries rather than all being banks. And the list is not fixed: companies are added and removed over the years as businesses grow and shrink.

Which is worth pausing on. An index quietly drops its failures. A company that collapses leaves the list; a company that grows joins it. So an index is always a list of survivors, and that flatters it a little. Nobody is cheating — but it is a real reason an index looks steadier over decades than the individual companies inside it ever were.

Bigger companies count more

Not every company in an index counts equally. They are weighted by size, so a very large company moves the index far more than a small one.

This is deliberate. If a huge company and a tiny one counted the same, the index would swing on businesses that hardly anybody owns. But it has a consequence, and it is the point of this chapter.

The consequence: the index can rise while most companies fall

Suppose an index has ten companies. Two of them are enormous. Eight are small.

On Tuesday, the two big ones rise sharply. The other eight all fall.

The index goes up. Eight companies out of ten fell, most investors had a bad day, and the headline still says "market rises".

That is not a trick or a scandal. It is arithmetic — the two large companies carry more weight than the eight small ones combined. But it means the headline number can genuinely disagree with almost everybody's experience.

So learn to ask one question

Whenever you hear that the market is up, ask:

Is the gain broad, or concentrated?

Broad — most companies rose, most sectors rose. The headline matches the reality. Concentrated — a few large companies did all the lifting. The headline is true and misleading at once.

You can check it. Open the sector view in this app and look at how many sectors are green against how many are red. Two sectors up and six down, on a day the index rose, tells you immediately which kind of day it was.

That check takes about ten seconds, and it is the difference between reading a headline and understanding it.

About the number itself

This chapter deliberately does not tell you what the Sensex is at today. Any figure printed here would be wrong by the time you read it. Open the live block in the app — that number is today's, and this text will still be true next year.

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Worked example

An index with five companies. Weights are set by size.

Add the contributions:

`` +200 + 90 − 40 − 60 − 50 = +140 divide by the total weight → +140 ÷ 100 = +1.4% ``

The index rose 1.4%.

Now count the companies. Three of the five fell. If you owned C, D or E — and most people own the smaller companies, not only the giants — you lost money on a day the news called good.

Work it once more with A moving −5% instead of +5%, and watch the same index turn negative while nothing else changed. That single company is worth more than C, D and E together.

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⚠️ The common mistake

"The market went up today, so my shares went up."

Your shares are not the index. The index is thirty companies, weighted by size; you probably own something else, or something smaller.

The index tells you what that particular group of thirty did. It tells you nothing about what you own unless you happen to own exactly those thirty in exactly those proportions — which almost nobody does.

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Live block

The sector view. Open it beside this chapter: the question "broad or concentrated?" is answered by counting green sectors against red ones, and a student who does that once will do it for life.

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Check yourself

1. In one sentence, say what an index is for. Do not use the word "market".
2. An index has three companies with weights 50%, 30% and 20%. They move +4%, −2% and −5%.

Work out the index move. Show your arithmetic.

1. Using your answer to question 2 — did the index rise or fall, and how many of the three

companies fell? What does that tell you?

1. The index rises 2%. Six sectors are red and two are green. Is this gain broad or

concentrated, and how did you decide?

1. A company in the index collapses and is removed from the list. Explain why this makes an

index look better over ten years than the companies inside it actually did.