ETF & Index Fund Guide
A comprehensive educational reference covering how exchange-traded funds and index funds work, their types, costs, and differences.
What Is an ETF?
An exchange-traded fund (ETF) is a type of investment fund that holds a collection of underlying assets — such as stocks, bonds, or commodities — and is traded on a stock exchange, much like an individual share.
When an investor buys one share of an ETF, they gain proportional exposure to all of the assets held within that fund. This makes ETFs an efficient way to achieve broad diversification in a single transaction.
Unlike traditional mutual funds, ETF shares can be bought and sold throughout the trading day at market prices that fluctuate continuously. This intraday liquidity is one of the defining characteristics that distinguishes ETFs from older fund structures.
Key Characteristics
Types of ETFs
ETFs are available across a broad range of asset classes and strategies. Understanding the main categories helps clarify the landscape of available products.
Equity ETFs
Track indices or segments of stock markets. Subcategories include broad market, sector, regional, country-specific, dividend, and factor-based (smart beta) equity ETFs.
Fixed Income ETFs
Hold bonds or other debt instruments. Includes government bond ETFs, corporate bond ETFs, high-yield ETFs, and inflation-linked bond ETFs across different durations.
Commodity ETFs
Provide exposure to physical commodities such as gold, silver, oil, or agricultural products, either through direct physical ownership or derivatives contracts.
Multi-Asset ETFs
Combine multiple asset classes within a single fund, often following a target allocation such as 60% equities and 40% bonds. Sometimes called allocation or balanced ETFs.
Inverse & Leveraged ETFs
Designed to deliver multiples of — or the inverse of — daily index returns. These are complex instruments primarily used for short-term trading and carry significant risks.
Real Estate ETFs (REITs)
Track indices of real estate investment trusts, providing exposure to property markets without direct ownership of real estate assets.
What Is an Index Fund?
An index fund is any investment fund — whether an ETF or a traditional mutual fund — that passively tracks a financial market index. Its objective is not to outperform the market (as an actively managed fund would attempt), but to replicate its returns as closely as possible.
The concept was popularised by John Bogle, who launched the first retail index mutual fund in 1976. The core argument for indexing is well-established in academic finance: because active managers collectively hold the market minus their costs, the average active fund must underperform an index fund tracking the same market.
Index funds can be structured as traditional mutual funds (priced once daily at net asset value) or as ETFs (traded continuously throughout the day). Both can track the same index, but they differ structurally in how investors access them.
Common Indices Tracked
ETF vs. Index Fund: Key Differences
While both ETFs and index mutual funds can track the same benchmark, there are important structural and practical differences between the two structures.
| Feature | ETF | Index Mutual Fund |
|---|---|---|
| Trading | Continuously on exchanges throughout the trading day | Once daily at end-of-day net asset value (NAV) |
| Minimum investment | Cost of one share (often <$100) | Often $1,000–$3,000 or more |
| Transaction costs | Bid-ask spread + any brokerage commission | None (typically) at fund level |
| Expense ratios | Often very low (0.03%–0.20%) | Also low (0.03%–0.50%) |
| Tax efficiency | Generally higher | Moderate (may distribute capital gains) |
| Fractional shares | Available at some brokers only | Typically available |
| Automatic investment | Less straightforward | Usually easy to automate |
| Transparency | Daily holdings disclosure | Quarterly disclosure (typically) |
Understanding Costs
Costs are one of the most important factors in evaluating index funds and ETFs. Because passive funds aim only to match — not beat — an index, any cost directly reduces the return delivered to investors.
Expense Ratio (TER / OCF)
The expense ratio (also called Total Expense Ratio or Ongoing Charges Figure) is the annual fee charged by the fund, expressed as a percentage of assets. For a broad equity ETF, this is typically 0.03%–0.20% annually. On a £10,000 investment, a 0.10% expense ratio costs £10 per year.
Tracking Difference
Tracking difference measures how closely a fund's actual return matched its benchmark index return over a period. A fund with a lower expense ratio may still have a larger tracking difference due to transaction costs, sampling methods, or securities lending income. It is often a more complete cost measure than the expense ratio alone.
Tracking Error
Tracking error measures the volatility (standard deviation) of the gap between a fund's return and the index's return. A lower tracking error indicates more consistent replication. It is distinct from tracking difference, which measures the average gap rather than its consistency.
Typical Expense Ratios by Category
Broad US or global equity index ETFs
Broad bond index ETFs
Emerging market ETFs
Sector or thematic ETFs
How ETFs Are Created and Redeemed
One of the most distinctive features of ETFs is their creation and redemption mechanism, which involves a class of large institutional investors known as authorised participants (APs).
Authorised Participant assembles the basket
When demand for an ETF rises, an authorised participant — typically a large bank or broker-dealer — assembles the underlying securities that the ETF holds, in the correct proportions.
Basket is exchanged for ETF shares
The AP delivers the basket of securities to the ETF provider and receives newly created ETF shares in return. This is an in-kind exchange — no cash changes hands at this stage.
AP sells ETF shares on the exchange
The authorised participant then sells these new ETF shares to investors on the open market, profiting from any spread between the NAV and the market price.
Arbitrage keeps prices aligned
If the ETF's market price diverges from the NAV of its underlying holdings, APs can profit by creating or redeeming shares. This arbitrage mechanism keeps ETF prices close to their fair value.
Replication Methods
ETFs do not all hold their underlying assets in the same way. Understanding replication method is important for assessing a fund's complexity and counterparty risk profile.
Full Physical Replication
The fund holds all securities in the index in the exact same proportions. This provides the closest tracking but can be operationally complex for large indices.
Sampling (Optimisation)
The fund holds a representative subset of index securities, chosen to replicate the index's characteristics without holding every constituent. Common for indices with thousands of securities.
Synthetic Replication
The fund uses derivatives (typically total return swaps) to replicate index returns, rather than holding underlying securities directly. Introduces counterparty risk but can be more efficient in some markets.
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