A stock index tracks the combined performance of a group of companies — for example the S&P 500 tracks 500 large US companies. Instead of buying 500 individual stocks, traders can speculate on the index as a single instrument, gaining broad market exposure with one position.
How Indices Are Built
Most major indices use market-cap weighting: companies with larger total share value move the index more. A few, like the Dow Jones, use price weighting, where a stock's raw share price (not its size) determines its influence — a quirk that makes the Dow behave differently from cap-weighted indices.
| Weighting | Example index | Effect |
|---|---|---|
| Market-cap weighted | S&P 500, DAX | Big companies (Apple, SAP) dominate moves |
| Price weighted | Dow Jones (US30) | High share-price stocks dominate regardless of size |
| Equal weighted | Some sector ETF indices | Every constituent counts the same |
Cash vs Futures CFDs
Brokers offer indices as either 'cash' CFDs (tracking spot fair value, adjusted daily for financing) or 'futures' CFDs (tracking a specific futures contract with its own expiry and roll dates). Cash CFDs suit intraday traders because pricing is continuous and simple; futures CFDs suit swing traders wanting exposure without daily financing drift.
Trading Hours
Index CFDs typically trade far beyond the underlying exchange's regular hours. US indices (US30, NAS100, SPX500) often trade around 23 hours a day via futures markets, while European indices (DAX, FTSE) have a defined cash session roughly 07:00-21:00 UTC with extended lower-liquidity hours either side.
Why Traders Use Indices
- ▸Diversification in one instrument — no single-stock earnings risk.
- ▸High liquidity on major indices with tight spreads during core hours.
- ▸Clear macro narrative — indices react to rates, growth data, and risk sentiment.
- ▸Trending behavior — indices historically drift upward over long periods.
Key Risks
- 1.Overnight gap risk — indices can gap sharply at the cash open after news.
- 2.Earnings season volatility — a handful of mega-cap earnings can swing cap-weighted indices.
- 3.Correlation clustering — most indices move together in broad risk-off events, reducing diversification when you need it most.
Trading an index means trading a country or sector's overall economic story, not one company. Interest rate decisions, GDP releases, and geopolitical headlines usually matter more than any single earnings report.
Typical Volatility
Major indices typically move 0.5%-1.5% on a normal day, with larger moves (2%+) around central bank decisions, major earnings clusters, or macro shocks. The NAS100 (tech-heavy) is usually the most volatile of the US majors, while the DAX tends to sit between the calmer FTSE and the more volatile US indices.