Not my words. Cut and pasted from the bogleheads forum. Another doc actually. His post is two years old. Agree with almost all:
BONDS WHAT THE DATA SHOW:
1. Risk taken in the bond arena is not rewarded in terms of lower credit quality. This means that you should stick to treasuries, AA/AAA or pre-refunded municipals, noncallable FDIC insured CD's, savings bonds, all noncallable or as uncallable as possible (many munis are callable just the last year, this should not be a big deal). Avoid lower grade munis, corporates of any kinds, GNMA's, callable bonds, junk bonds, emerging bonds or foreign bonds. This is because while over certain periods these assets may seem to do well, over the long term the risk has not been rewarded and includes equity correlation. Ever see Total Bond Market? It has done better and is more efficient than a comparable intermediate treasury only bond fund. However, when you mix the respective bond funds with equity, the 5 year treasury/equity portfolio outperforms because the treasuries are less correlated. The corporate and GNMA part of total bond tends to lose value at the same time that equities do, thereby depriving you of the rebalancing bonus and losing portfolio efficiency.
2. Durations less than a year have no equity correlation. Durations up to 5 years start to increase equity correlation. Durations past 5 years strongly correlated with equity. Sweet spot for treasuries is never more than 5 years, municipals is about 6-7 (Depending on curve).
3. Extending duration from 5 years, which is generally the sweet spot for treasuries, to 20 years has over the last 60 years only increased real returns 40 basis points. That is a high price to pay for the volatility of longer bonds.
4. You can extend duration from 1-3 years to say 3-7 years if you make an allocation to commodities of say 5% of your AA and rebalance faithfully. This actually increases portfolio efficiency because it exploits the longer bonds capital appreciation in falling rate environments when stocks otherwise tank, but hedges the longer bonds volatility because commodities are very volatile and will upsurge if interest rates were to rise.
5. Long bonds? Actually, long bonds produce a more efficient portfolio when you are talking about a portfolio that is heavily weighted towards equity. Therefore, if you have 70% equity or greater, the volatiliy of the portfolio is controlled by the equity anyhow so extending the bond duration will not appreciably worsen your overall volatility, and actually helps offset risk somewhat. Sharpe ratios (the measure of reward vs amount of risk taken) is actually better in heavily equity portfolio with 20 year average duration treasuries than with only 5 years. However, if you have a portfolio that relied heavily on fixed income, a duration of greater than 5 years is hazardous to its health and should be avoided.
6. If 5-7 year yields don't appeal to you, you can safely reach for yield in the following manner: by building a ladder. Buy equal amounts of dollar weighting in debt maturing 10% each in every one of the next 10 years: 10% in 2012, 10% in 2013 etc. When the bonds mature, you buy the next 10 year bond that will fit (10 year muni, 10 year treasury, 10 year TIP whatever). This way, you will have a bond portfolio with an average maturity of 5 years whose overall performance characteristics will match that of a 5 year bond but once you set up the ladder, all new bonds or bonds you roll over will always get the 10 year yield. It's one of the few free lunches in fixed income investing. The other is the volume premium in municipals. Stick to odd lots, lots of 5 or 10 bonds. They will carry higher yields because of the liquidity premium. If you have to sell before maturity on these highly illiquid small lots that no one wants, you will get a huge haircut. If you keep, however, you can get some great deals. I have recently gotten 4% yield on a 10 year pre-refunded, and 1.5% yield going out 18 months, both pre-re's backed by treasuries.
7. TIPS help portfolio efficiency due to their safety, and you can safely extend up to about 15 year duration because you don't take inflation risk, even longer, but given low yields recently it would be more prudent to limit maturities to no more than 15 years. If you buy the individual tips and hold to maturity, they can never go to below par value and therefore also provide good deflation protection because no matter how much they and the dollar deflates, you might lose the CPI adjustment but never the coupon and never mature for less than 100% which is what you paid for it (more or less, some second auctions later in year have slightly higher or lower prices).
8. Consider having a good part of your bond allocation in your tax deferred be TIPS, and your taxable bonds be high quality munis that are preferrably pre-refunded. Also consider federal agency bonds and CD's in taxable for your very short term part of the ladder, as their after tax yield can, but not always are higher than the municipals.
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at this time I don't hold any TIPs because the real yields are too low for me. They are the only long term bonds that I would consider. I don't buy individual munis, only funds. Vanguard and Baird (BMBIX)- like the credit quality of the last one. Highest quality Munis are currently yielding about 95% of treasuries. Making them relatively cheap.