Market Indices
When a news headline says "the market fell today," it almost never means every single stock fell — it means some specific basket of stocks, tracked as a single number, moved down, and understanding what that basket actually contains changes how much weight you should give the headline.
What an index is
A market index is a basket of selected stocks (or other assets) combined into a single number, designed to measure the overall performance of a market, sector, or theme, so investors have a simple benchmark to track instead of following thousands of individual securities. Indices aren't things you can buy directly off an exchange floor — they're calculated figures, published continuously, that summarize how a defined group of stocks is performing as a whole. You gain exposure to an index in practice by buying an index fund or ETF (exchange-traded fund) built to track it, which we'll cover more in Unit 7.
STI, S&P 500, and MSCI World: what each represents
Different indices measure different slices of the investable world:
| Index | What it tracks |
|---|---|
| STI (Straits Times Index) | The 30 largest and most liquid companies listed on the Singapore Exchange, including DBS, OCBC, and Singtel |
| S&P 500 | 500 of the largest publicly listed companies in the United States, widely used as the benchmark for the overall U.S. stock market |
| MSCI World | Thousands of large and mid-sized companies across 23 developed markets globally, used as a benchmark for developed-market stocks worldwide |
Each index is really answering a slightly different question: the STI answers "how is the Singapore stock market doing," the S&P 500 answers the same question for the U.S., and MSCI World zooms out to answer it for developed economies as a whole. Comparing your portfolio's performance to the "wrong" index — say, comparing a Singapore-focused portfolio to the S&P 500 — tells you very little, because you'd be measuring against a completely different set of companies and economic conditions.
Market-cap weighting: why Apple matters more than most companies
Most major indices, including the S&P 500, are market-cap weighted — meaning each company's influence on the index's overall movement is proportional to its market capitalization (share price × total number of shares outstanding, i.e. the total market value of the company), not simply one company, one equal vote. A company worth $3 trillion will move the S&P 500 far more than a company worth $10 billion, even though both are just "one of the 500" companies in the index.
This is precisely why headlines sometimes note that "a handful of mega-cap tech stocks are responsible for most of the S&P 500's gains this year" — when a small number of companies are worth trillions of dollars each, their price swings can single-handedly move the entire index, even if the other 490-odd smaller companies in the index barely moved at all. It also means the index isn't a neutral, evenly-spread bet on "the market" as a whole — it's tilted heavily toward whichever companies happen to be largest at the time.
Indices as benchmarks: what "beating the market" means
Indices serve as benchmarks — reference points used to judge whether an investment strategy or fund manager actually added value, by comparing its returns to what a comparable index achieved over the same period. "Beating the market" specifically means outperforming the relevant benchmark index over a given period, after accounting for fees. This standard exists because it's easy for a fund to sound impressive by saying "we returned 12% this year," but that claim means very little without context — if the relevant index returned 18% over the same period, the fund actually underperformed, despite the positive-sounding number. A fund manager who charges high fees but consistently fails to beat a simple, low-cost index fund over long periods is arguably not creating value for their investors, which is one of the central arguments behind the popularity of low-cost index investing.
If the S&P 500 is up 10% but your portfolio is up 7%, did you do well?
Reveal Answer
Relative to the benchmark, no — your portfolio underperformed the S&P 500 by 3 percentage points over that period, which is the standard way "did well" gets measured in investing. A 7% return is still a positive, real gain in absolute terms, and it might even be a perfectly sensible outcome if your portfolio was deliberately built to take less risk than the S&P 500 (for example, holding a mix of stocks and bonds rather than being fully invested in stocks), since taking less risk should generally come with a lower expected return. But if your portfolio holds similar risk and similar types of assets to the S&P 500 and still returned less, that gap represents value lost — whether from fees, poor stock selection, or bad timing — that a simple, low-cost S&P 500 index fund would have avoided entirely.