Epps24 February 2013 11:54
"As a simple guide, when rebalancing into bonds, normally i would keep things simple and buy the exact same bond if difference in maturity date between existing and new 30 years bond is less than 5 years apart. If an investor is more sophisticated, he/she can also analyse if the newer 30 year bond is offering much higher yield and then choose accordingly. Below is my layman answer to your second question, hope you have some knowledge of bonds to understand it:
If the two 30 years bonds are issued 6 months apart, then logically speaking their price will be different by a few dollars at least.
The price difference is not important, what is important is that the two bonds will have same yield and the same percentage increase/decrease in returns say 2 years later - since both bond are half year apart only, practically they are almost the same bonds. If one bond is slightly cheaper than the other, more people will buy the cheaper bond and push up its price.
So when is one bond considered cheaper than the other? When both bonds have very close and similar maturity dates, their yield should be practically the same, so in this case, the bond with higher yield should be the cheaper bond.
First, you should know that bond price and bond yield are mathematically and inversely correlated - when bond price goes up, bond yield goes down - when more people buy higher yield bond, its price goes up and yield goes down and become more expensive. Second, both bonds with similar maturity will be issued with different yield, so on a short term basis, they can have different prices but have same yield. Third, if one bond has higher yield than another, more people will buy the higher yielding (cheaper) 30 year bond, push up its price, and lower its yield, thus ensuring both bonds have similar yield.
Practically speaking, for both bonds with similar maturity dates (6 months apart only), you should be using bond yield to compare both bonds instead of price. In this case, either one of the 2 bonds can be bought because they should have practically same yield and same magnitude of price increase/decrease in the future. In which case, you probably want to stick to always buying the same bond as previously bought so that it is easier to track performance.
If both 30 years bonds have big difference in their time till maturity, you may wish to calculate which bond has higher yield and act accordingly. On the other hand, we as average investor can also keep things simple by investing in the same bond, then when the bond is approaching 20 years remaining till maturity, then we can sell off all these 20~21 years bonds and buy all into latest 30 years bonds. As average investor, we do not have to analyse about bond yield in so much details just to squeeze that 0.3~0.5% more in long term returns. Either way, as long as you buy long term bonds (20+ to 30 years), any maturity period between 20+ to 30 years will work, so you don’t have to be so particularly precise about choosing based on bond pricing or yield. "
Taken from Epps blog in the comments section. ( Cant link it for some reason)
Hope you dont mind Epps!
Cheers