What it means
An index takes a defined group of securities, applies a weighting rule and expresses the result relative to a starting value, usually set at a round number such as 100 or 1,000 on a chosen base date. From then on, movements in the index show how the basket as a whole has performed since that date.
Weighting is what makes indices differ from one another. Market capitalisation weighting gives the biggest companies the largest influence, price weighting uses the share price alone, and equal weighting gives every constituent the same pull regardless of size.
Indices matter to businesses and investors for three reasons. They summarise sentiment in a single figure, they act as the benchmark a fund manager is judged against, and they can be tracked directly through funds that hold the constituents in the same proportions as the index itself.
Constituents change over time. Index providers run periodic reviews that add companies which have grown and remove those which have shrunk, which quietly flatters long-run index performance because failing companies drop out along the way.
The nuance that trips people up is what the index actually measures. A price index excludes dividends, so it understates the total return an investor would have earned, whereas a total return version reinvests dividends and gives the fuller picture.
Coverage is the other thing worth checking before quoting an index. Some cover a handful of very large companies, some cover thousands across many countries, and some track only a single sector, so two headlines about the same day's trading can point in opposite directions quite legitimately.
In practice
Real-world examples.
Example
A pension trustee compares the fund manager's 7% annual return against a broad equity index that returned 9%. The manager charged active fees but delivered less than the benchmark, which prompts an uncomfortable review meeting.
Example
A treasury team benchmarks its corporate bond holdings against a bond index to check whether losses in a bad quarter were company-specific or simply the whole market moving.
Example
A retail investor buys an index tracker fund rather than picking shares. The fund holds the constituents in index proportions and charges a low fee, so the return closely follows the index minus costs.
Formula
Calculation
Capitalisation-weighted index = (current total market value of constituents / base period total market value) x base index value
A small exchange builds an index from three listed companies, with a base date total market value of $80,000,000,000 and a base index value of 1,000.
At today's close, the three companies are worth $60,000,000,000, $30,000,000,000 and $10,000,000,000, giving a current total of $100,000,000,000.
Index level = ($100,000,000,000 / $80,000,000,000) x 1,000 = 1.25 x 1,000 = 1,250
The index therefore stands at 1,250, meaning the basket has risen 25% since the base date. Notice how weighting matters: the largest company represents $60,000,000,000 out of $100,000,000,000, or 60% of the index, so a 10% move in that one share alone would shift the index by 6%, while the same 10% move in the smallest constituent would shift it by only 1%.Case study
Seen in the real world.
Aldergate Capital is an illustrative and entirely fictional boutique fund manager used here to show how index benchmarking bites. Aldergate marketed a fund that returned 11% over a year, presenting the number to clients as strong performance in a difficult market.
A pension client checked the figure against the relevant index, which had returned 14% over the same period. Aldergate's fund had underperformed by 3 percentage points before fees and by roughly 4 after them, so a $50,000,000 mandate had ended up around $2,000,000 behind a cheap tracker.
The fictional outcome was a change in reporting rather than in strategy. Aldergate began quoting returns against the index in every client letter, which was uncomfortable in weak years but built more trust than presenting a positive number with no benchmark beside it.
Watch out
Common mistakes.
- Reading the index level as a price or a value in dollars. An index at 4,200 is simply 4.2 times its base value and represents no purchasable amount.
- Assuming an index represents the whole economy. Most indices cover listed companies only, so private firms, which employ a large share of workers, are absent entirely.
- Comparing a portfolio against an index that holds very different assets. A small-company fund judged against a large-company index will look wrong in both good years and bad.
Questions
People also ask.
Why do two indices covering the same market move differently?
Because they use different constituents, different weighting rules and sometimes different treatment of dividends.
Can you invest in an index directly?
Not in the index itself, but tracker funds and exchange traded funds hold the constituents and aim to reproduce its return closely.
What does it mean when an index is rebalanced?
The provider resets weightings or changes constituents according to its published rules, and tracker funds then trade to match the new composition.
From the founder's library
Take it further with the book.
Build your financial confidence beyond this definition. Shihan's full-length guide, Accounting Fundamentals, takes the same plain-English approach and turns it into a complete, practical playbook for non-finance managers, business owners and students - with chapter-end quiz answers and presentation slides included.
25% off with code MMHQ25, applied at checkout. Priced in USD - checkout may show the equivalent in your local currency.
View the book and save 25%