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Market Indices

In short

Basket-based measures of a market segment — SPX, NDX, DJIA, R2K

An index tracks a group of stocks to gauge how a market or sector is doing. The S&P 500 (^GSPC) tracks 500 large US companies, NASDAQ-100 (^NDX) tracks 100 big tech/growth names, Dow (^DJI) tracks 30 blue chips, Russell 2000 (^RUT) tracks 2,000 small caps. Index ≠ ETF — ETFs like SPY are investable proxies, but the index is the underlying reference.

A market index is a rules-based portfolio whose value represents a benchmark. Most are market-cap weighted (S&P 500, NASDAQ-100) so larger companies move it more; the Dow is price-weighted (higher-priced stocks dominate). Indices are used as performance benchmarks, economic signals, and underlying references for index funds, ETFs, and derivatives. Pay attention to sector concentration — the S&P 500 is ~30% technology.

Related concepts

  • Market Capitalization — Market cap = share price × shares outstanding. Apple at $200/share with 15B shares = $3T market cap. Large caps (>$10B) are stable. Small caps (<$2B) are riskier but have more growth potential.
  • Market Regime — Markets behave differently in different regimes. A bull regime rewards growth and risk; a bear regime rewards defensives and cash; high-volatility regimes punish leverage. We combine VIX, the 10Y–2Y yield spread, and credit spreads to classify the current regime so you can size positions and pick strategies accordingly.
  • VIX (Volatility Index) — VIX is called the 'fear gauge.' When investors are scared, they buy options to protect themselves, which pushes VIX up. VIX above 30 means real fear in the market. Below 15 means complacency. Extreme fear is often a contrarian buy signal.
  • Sector — Sectors group companies by industry: Technology, Healthcare, Financials, Energy, etc. Diversifying across sectors means a crash in tech won't destroy your whole portfolio. Different sectors perform well in different economic conditions.