Holding Your Bond Fund for the Duration

Bond index funds have a place in almost all portfolios, even in a low-rate environment. However, it’s important to match the right bond fund to your investment goals. To do that you need to know two important details. You can usually find both of these numbers on the web page or fact card of any […]



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Bond index funds have a place in almost all portfolios, even in a low-rate environment. However, it’s important to match the right bond fund to your investment goals. To do that you need to know two important details. You can usually find both of these numbers on the web page or fact card of any bond mutual fund or ETF.

The first is the weighted average term to maturity of the bonds in the fund. For example, the iShares DEX Universe Bond Index Fund (XBB)—which tracks the most popular fixed-income benchmark in Canada—is about half short-term (one to five years to maturity), one quarter intermediate-term (five to 10 years) and one quarter long-term bonds. The weighted average term to maturity of all the bonds in the fund is 9.3 years.

This number is important, because the fund will behave much like an individual bond of about this same maturity. Sure enough, if you look up the current yield on Government of Canada 10-year bonds you’ll find it is 3.07%, almost identical to XBB’s current yield to maturity (3.03%). Now you know that XBB will be sensitive to the prevailing interest rate on 10-year bonds: if this yield goes up, the fund’s value will fall. Other interest rates—such as the Bank of Canada’s overnight rate that you keep hearing about on the news—are pretty much irrelevant.

The second key figure is your bond fund’s weighted average duration. This tells you the fund’s sensitivity to interest-rate movements: the longer the duration, the more your fund will lose if rates go up.

Duration is a calculated with a complicated formula that considers a bond’s term to maturity and its coupon. The important idea is that the longer the maturity, or the lower the coupon, the longer the duration. This is why short-term bonds are less sensitive to interest rate swings, and why higher-yielding corporates are less vulnerable than government bonds:

Ticker Term Coupon Duration
iShares DEX Short Term Bond XSB 2.9 3.62% 2.7
iShares DEX All Corporate Bond XCB 8.3 5.21% 5.5
iShares DEX Universe Bond XBB 9.3 4.41% 6.3
iShares DEX All Government Bond XGB 9.5 4.00% 6.6
iShares DEX Long Term Bond XLB 22.8 5.72% 13.7
iShares DEX Real Return Bond XRB 20.9 3.40% 16.2

As you can see in the table, XBB (and similar broad-based funds) have a duration of just over six. That means if the relevant interest rate rises one percentage point—remember, in this case it’s the yield on nine- or 10-year bonds—then the fund can be expected to fall in value by about six percentage points.

And the bond played on

There are a couple of subtleties to be aware of here. First, interest rates at the long end of the yield curve tend to be less volatile than short-term rates: it’s unusual for 10-year bond yields to move more than one or two percentage points in a year. So the chance of a fund like XBB suffering double-digit losses in any given year is remote—at least if history is any guide.

The iShares DEX Long Term Bond Index Fund (XLB) looks even scarier with its duration of almost 14: a 2% jump in the yield on 20-year bonds would theoretically mean the fund’s value would decline by some 27%. But this has never happened. Since 1948, the worst one-year return on the DEX Long-Term Bond Index was –8.9% in 1956, followed by –7.4% in 1994. In the U.S., long-term bonds have seen just one double-digit decline in 85 years (that happened in 2009).

Another often overlooked point is that rising interest rates have a silver lining: new bonds are issued with higher coupons, and this will eventually lead to more income. (Doesn’t it strike you as odd that fixed-income investors complain about low rates while also worrying they might go up?) As bonds in a fund mature, the proceeds are reinvested in higher-yielding bonds that help offset the price declines. That’s why bonds recover from bear markets much faster than stocks do.

Stay in for the duration

Which leads us to the key message for investors: as long as your time horizon is at least as long as the duration of your bond fund, you won’t lose any capital.

You’ve probably heard people say they prefer individual bonds to bond funds, because as long as they hold on until maturity, they won’t lose principal. Well, the same is true if you hold a bond fund for a period equal to its duration. You can be sure that XBB will not lose value over any period longer than 6.3 years: any price decline from rising interest rates will be offset by higher coupons within that time frame. In fact, history suggests the recovery is likely to be more swift than that: even a three-year period of negative bond returns is extremely rare.

So, if you’re saving for a child’s education with a three-year time horizon, steer clear of XBB and choose a fund with a duration less than three—or just put your money in a GIC or high-interest savings account. But if you’re investing for a retirement that’s 10, 20 or 30 years down the road, a broad-based bond index fund should still be a core holding in your portfolio.

3 comments on “Holding Your Bond Fund for the Duration

  1. Hi CCP,
    what about bond diversification??and the selection of both government and corporate bonds to mitigate against the risk of changing yields? From March to May of this year we saw the values of bond funds drop slightly, and then they spiked back up when equities looked unfavourable in June… just curious about the reasoning behind this. Notice that residential mortgage rates have relation to those same 10 year government bond yields too. Sorry for being wordy…


  2. @Adrian: Thanks for the comment. Indeed, one of the most important reasons to hold bonds is that they are a portfolio diversifier that helps to balance equity risk. That's another reason why it's dangerous for investors to think that dividend-paying stocks are a substitute for bonds. They are not.

    Residential mortgage rates have no meaningful connection to 10-year bond yields. If they happen to move in concert with each other over a certain period, that's a coincidence.


  3. Hello, rather late after your post to seek advice but wanted to ask if this is an opportune time to buy U.S. corporate bonds, in your opinion. I am looking at BMO's "ZHY" which is yielding north of 8 pc and hedged to CDN dollar.

    Many thanks,



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