1. How they are managed 

An ETF portfolio manager (PM)’s number one goal is to track the performance of the fund’s target index as closely as possible, after fees and expenses. The difficulty of this task can vary greatly depending on how accessible the securities in the index are. For example, it’s relatively easy to trade the large cap stocks in the S&P 500 Index, whereas it’s harder to trade the less liquid stocks in the MSCI Frontier Market Index. Tracking a bond index adds another layer of complexity. Some bond indexes are huge—think hundreds or even thousands of bonds. And since bonds may be less liquid than stocks, it’s often impossible for an ETF to own every security in a given index—some of those bonds are simply unavailable. Instead, bond ETF managers use a “sampling” approach where they try to replicate the risk and return characteristics of the index using a smaller portfolio of available bonds. Tracking a bond index can be a challenge, particularly in a highly illiquid sector such as high yield. The PM of the ETF is constantly working to reduce portfolio tracking error vs. the fund’s index. And since reputable ETF providers leverage economies of scale and bond desk relationships in order to facilitate trades in illiquid securities, investors actually get exposure to a wider variety of bonds than they would likely be able to access on their own. Basically, the bond ETF does the legwork of tracking down the bond and seeking to ensure a fair price for you.

2. How they calculate underlying value

Another key difference between bond ETFs and equity ETFs is the way that they calculate underlying value. Since stocks trade on an exchange, the public can see each stock’s current price at any point during market hours, as well as a closing price at market close. ETF providers use stocks’ prices to calculate an ETF’s intraday underlying value throughout the trading day, and the closing net asset value (NAV) of an equity ETF is typically very close to the ETF’s closing price. Bonds trade OTC, and there’s typically no central market where investors can see where bonds were bought and sold. At the same time the majority of bonds do not trade every day and so their value must be estimated based upon available market information. This means that the calculation of a bond ETF’s underlying value is going to be less precise than a stock ETF’s underlying value. As a result, bond ETFs tend to experience more premiums and discounts, or deviation between the closing ETF price and the closing NAV. But while an ETF’s NAV is the best estimate of that fund’s underlying value, it’s still just an estimate—especially for bonds. It’s not an actionable price that investors can use to transact in the portfolio of underlying securities. The reality is that market price of a bond ETF represents the price at which the underlying bonds can actually be traded at any given moment. Often when we talk about how innovative bond ETFs are, this is what we’re referring to—the bond market simply didn’t have this kind of price transparency before bond ETFs came along.

If history isn’t your subject, just keep in mind that despite their differences both stock and bond ETFs provide an efficient way to invest in markets. And that is one lesson worth remembering.

Matt Tucker, CFA, is the iShares Head of Fixed Income Strategy and a regular contributor to The Blog.