How An ETF Trades During The Day

An exchange-traded fund is a pooled portfolio that trades on an exchange like a single share. That dual nature explains most of what looks strange about how these funds behave during a trading day.

Two prices exist at the same time

Every fund has a net asset value, which is simply the value of everything it holds divided by the number of units in issue. That figure is the fund's underlying worth.

An exchange-traded fund also has a market price, set by buyers and sellers on the exchange throughout the session. The two numbers are related but they are produced by different processes.

The market price can sit slightly above or below net asset value at any moment. A persistent gap would be a problem, and the structure contains a mechanism designed to prevent one forming.

Creation and redemption close the gap

Large institutions known as authorised participants can exchange a basket of the underlying securities for new fund units, or hand units back and receive securities. This happens directly with the fund.

If the market price drifts above net asset value, creating units and selling them is profitable, and that selling pushes the price back down. The reverse works when the price drifts below.

The gap is therefore policed by ordinary self-interest rather than by any rule requiring the prices to match. It closes because someone can earn a small margin by closing it.

The spread is a real cost of dealing

On the exchange, market makers quote a price to buy and a slightly higher price to sell. The difference between the two is the bid-offer spread.

That spread is paid by whoever crosses it, and it is separate from the fund's ongoing charge. It reflects how easily the market maker can hedge the position.

Funds holding widely traded shares generally show narrower spreads than funds holding thinly traded bonds or shares in smaller markets. The underlying market's liquidity passes through to the wrapper.

Underlying markets may be closed

A fund listed in one time zone may hold assets that trade in another. When those markets are shut, the fund's quoted price reflects estimates rather than live prices for its holdings.

Market makers price in what they think has happened since the underlying market closed. The fund can therefore move while the securities inside it are not trading at all.

This is why an apparent premium or discount can appear around the open and close. It is often a timing artefact rather than evidence that the fund is mispriced.

Stress reveals the plumbing

During disorderly markets, the underlying securities can become hard to trade, particularly in credit. The fund's price then reflects what dealers will actually transact at rather than stale valuations.

Observers sometimes read this as the fund breaking, when it is closer to the fund revealing where the real market sits. The wrapper is showing information the underlying prices lag.

Listing rules, disclosure requirements and the treatment of these funds vary by jurisdiction and change over time, so the documentation for a specific fund is the authority on how it operates.