3. Bond's Yield to Maturity
A bond's current yield to maturity indicates the return you could get by investing in the bond today and hold until its maturity date. It has been proven as a very good predictor for bonds for every 10-year period since 1906.
For example, today the 10-year Treasury note's yield is only around 1.4%, it indicates trouble for long term investors, especially retirement investors, if the Fed could achieve its inflation target of 2%.
However, you probably would feel better if you know there are over 10 trillions of world's sovereign bonds currently have negative yields, especially after the Brexit event.
If you want to achieve higher return, you would have to take more risk. For example, you could invest in high-quality corporate bonds currently have around 3% yield. iShares iBoxx $ Investment Grade Corporate Bond ETF (LQD) is a good candidate, at least you could beat inflation.