Index fund
What is Index fund?
An index fund is a pooled investment vehicle designed to track the performance of a specified index before fees and implementation differences rather than select holdings through discretionary security forecasts. Investors therefore select both a fund and the index methodology embedded within it.
How index funds work
The fund may fully replicate index constituents, sample them, or use derivatives and other instruments. It follows published or licensed rules for eligibility, weighting, reconstitution, and corporate actions. Passive describes adherence to the methodology, not an absence of decisions: the index provider determines the market segment and rules, while the manager makes implementation, cash, trading, and tax choices.
Tracking and return
Tracking difference is the fund return minus index return over a period, while tracking error measures variability in that difference. Fees, taxes, withholding, cash drag, sampling, rebalancing, securities lending, transaction costs, and valuation timing contribute. A low expense ratio can coexist with poor tracking, and a favorable tracking difference can arise from lending income or tax treatment rather than manager skill.
Example
An index adds a security at Friday's closing price. A fund may trade before, at, or after that event to balance tracking and market impact. Its realized cost can differ from the theoretical index. If the fund samples illiquid bonds rather than holding every issue, small exposure differences may also create return deviations as yields and spreads move.
How to evaluate one
Read the index methodology and fund prospectus. Examine exposure, weighting, concentration, reconstitution, turnover, replication, derivatives, domicile, securities lending, tracking history, scale, and fees. Compare the fund with the exact net or gross index version it seeks to follow. Confirm point-in-time benchmark membership when evaluating historical claims and avoid assuming all products with similar names track identical universes.
Risks and practical checklist
Index funds bear every risk and valuation embedded in their benchmark, including concentration and mechanical exposure to overpriced securities. Rule changes, index-provider discretion, crowded rebalances, and fund closure can affect results. Check whether the exposure overlaps existing holdings and whether an alternative methodology is more suitable. Passive implementation reduces some manager-selection risk but does not remove market, liquidity, currency, tax, or behavior risk.
Also known as: passive fund, tracker fund
Sources and further reading
- Mutual Funds, Investor.gov, U.S. Securities and Exchange Commission