On Monday I looked at the relationship between an ETF’s daily trading volume and its liquidity. Unlike an individual stock, an infrequently traded ETF should not necessarily have a wide bid-ask spread. But low volume can cause some confusion for investors, and I’d like to share an example.
For a while now I’ve noticed that the Horizons S&P 500 (HXS) doesn’t always move in concert with its index. On Tuesday morning the S&P 500 was up about 0.5%, but my brokerage was showing HXS down about the same amount. Moreover, the quote said the last order was filled at $12.43, while the bid price was $12.46 and the ask was $12.48. That didn’t seem to make sense: the last price should be between the bid and ask, not several cents lower or higher.
I called Horizons to ask what was going on and got a helpful response from Jaime Purvis, Executive Vice-President, National Accounts. But first let’s review how ETFs are priced, and how this differs from mutual funds and individual stocks.
NAV versus market price
With a mutual fund, the manager determines the net asset value (NAV) per share once a day after the market closes. If you place an order during the day, it will be filled that afternoon, and you will always get the NAV price, regardless of whether you’re buying or selling. There’s no bid-ask spread, which is an often overlooked benefit of mutual funds.
Since ETFs trade throughout the day, their net asset value must be constantly updated while the market is open. Market makers post bid and ask prices to reflect that NAV, with a small spread that represents their commission. If the NAV is $20 per share, for example, the bid price might be $19.99 and the ask price $20.01. The most liquid ETFs have spreads of just a penny.
Contrast this with an individual stock, where there is no difference between NAV and market price. “The value of a stock is determined purely by its trading price, which is de facto its market value,” Purvis explains. “While this is a simple concept, it can be jarring for investors used to buying stocks, and for advisors who are used to buying mutual funds.”
Unsung heroes of arbitrage
Occasionally an ETF’s market price drifts away from the NAV, but this doesn’t usually last long, because arbitrageurs don’t allow it to. Say an ETF holding a basket of 50 stocks has a net asset value of $20 per share, but its ask price is just $19.75. An arbitrageur could buy units of the ETF and simultaneously sell its 50 underlying stocks to net a risk-free profit.
We’re accustomed to thinking of high-frequency traders (HFTs) as evildoers, but Purvis says they perform a useful role. “This is actually one of the unsung benefits of HFTs: they ensure fair bid-ask spreads on ETFs for individual investors. In doing so, they take the profit from arbitrage opportunities.”
So the ETF pricing mechanism is fair, regardless of trading volume. But infrequently traded ETFs can be puzzling for investors, as my experience with HXS shows. When I saw the fund apparently down 0.5% on a day when the S&P 500 was up, I wondered whether I could do a little arbitrage myself, picking up the ETF for what seemed like a 1% discount at $12.43. Just for fun I threw in a market order for a couple of shares—and it was promptly filled at $12.48. So much for my discount.
The price is right
The reason turned out to be simple: HXS has low trading volume and mine was the first order of the day. The price of the last trade ($12.43) was out of whack because it had been filled the day before. As soon as I placed my order, the market makers went to work and I got a price that was right around the NAV.
The lesson is that you should ignore the price of the last trade and look only at the bid-ask spread. “If it looks like you can get the ETF at a discount, or sell at a premium compared to its last price, think again,” Purvis says. “It’s just that investors are hardwired to look at the last price of a stock to determine the value of the security. This is not applicable to ETFs.”
Purvis has an important tip for anyone trading ETFs, whatever their daily volume: “Always use limit orders. Always, always, always. No exceptions. Market dislocations can happen from time to time—like the Flash Crash of 2010—so protect yourself by putting in the price you want to pay. If it’s reasonable, it will get filled. Don’t be afraid to move the limit order if the underlying securities move substantially.”
On Tuesday, had I placed a limit order to buy HXS at $12.43, it would have not have been filled, because it was well below the fund’s net asset value. I should have placed a limit order right at the ask price, perhaps a penny higher. That would have eliminated the surprise of having my order filled at a price five cents higher than the last guy’s.
Horizons offers some other ETF trading tips here.