An exchange-traded fund is a pooled vehicle that holds a basket of assets and trades on an exchange like a share. Most track an index, but the structure is used for many strategies.
How the structure works
Two mechanisms distinguish an ETF from an ordinary fund:
- Creation and redemption. Large institutions ("authorised participants") can deliver the underlying basket to the fund and receive ETF shares in return, or hand back shares for the basket. This arbitrage link is what keeps the market price close to the value of the holdings.
- Secondary-market trading. Everyone else buys and sells shares on the exchange, at whatever price the market sets.
Because of the first mechanism, an ETF can trade all day without the fund itself having to sell holdings to meet redemptions — most of the time. That link can weaken under stress.
Where the costs are
The stated expense ratio is the most visible cost and often the least important. The full picture includes:
- Expense ratio — deducted from the fund's assets, so it appears in performance rather than on a statement.
- Bid-ask spread — the gap between what buyers offer and sellers ask. Paid on every trade, invisible in the price chart, and much wider in thinly traded funds.
- Premium or discount — the difference between the market price and the fund's underlying value. Usually small; can widen when markets are dislocated.
- Tracking difference — the gap between the fund's return and its index return, after all costs. Over a holding period this is the number that actually matters.
Tracking difference vs tracking error
These are frequently conflated:
- Tracking difference is the cumulative gap in return. If an index returns 8% and the fund returns 7.7%, the tracking difference is 0.3 percentage points.
- Tracking error is the volatility of that gap around its average — how erratic it is.
A fund can have a small tracking difference and a large tracking error, or the reverse. For a long-term holder, tracking difference is usually the more consequential of the two, because it compounds.
Practical checks before choosing an ETF
- What exactly does it hold? Read the index methodology and the current holdings. Two funds with similar names can hold different things.
- How is it structured and where are the assets held? Physical replication and synthetic exposure are not the same risk. Assets held with a custodian are subject to that custodian.
- What is the dealing liquidity? Not just the fund's assets under management — its typical spread and volume on the exchange.
- What is the lending policy? Some funds lend securities to earn a little extra; that introduces counterparty exposure.
- Does the fund's index change rules often? A frequently rebalanced index can generate hidden turnover costs.
ETFs are a useful tool precisely because they can deliver broad, low-cost exposure. They are not automatically low-risk, and the label "index fund" says nothing about what the index contains.