Yield to Worst (YTW) is a tool used to measure the worst-case scenario with respect to the return on a given bond. It is important for bond investors to understand yield to worst because it provides a financial metric that indicates the worst potential return on the bond, allowing investors to make accurate predictions with the most up-to date information.

Yield to worst measures the lowest potential yield on a bond with an early retirement provision. This means that in the event of an early retirement, such as a call provision being exercised, the bondholder would receive no more than the yield calculated by the yield to worst calculation. Yield to worst is also referred to as yield to call as it is normally the same as the yield to call provision.

Yield to worst must always be less than yield to maturity since it gives the investor an estimate of the return in the event of an abbreviated investment period. This makes yield to worst an essential piece of information to consider when investing in a bond, as it gives investors a better understanding of the potential return on investment given different market conditions.

Yield to worst is calculated by adding the market price of the bond and the bond's remaining coupon payments. This is then divided by the years remaining in the bond’s life and multiplied by 100 to represent the interest rate as a percentage. Yield to worst is a handy tool that can give investors an estimate of the return they may receive if they invested in a bond with an early retirement provision.

In conclusion, yield to worst is an important metric to consider when investing in bonds. It provides investors with a measure of the lowest potential return that could be received on a bond with an early retirement provision. While it is not the most accurate predictor of potential returns, it does allow investors to make decisions based on the most up-to date information which ultimately leads to more judicious investments.