How the risk-free interest rate is determined

 US-PDGov

On the surface a risk-free interest rate is perceived to be the rate of return on an a financial allocation that has no financial risk associated with it. However, 'risk' itself is a word that has more depth to its meaning when coupled with financial instruments. This is because risk also includes the influence of variables such as devaluation from inflation, opportunity cost, and issuer default.

According to Aswath Damodaran of the New York University Stern School of Business, risk-free assets have no variance from the expected rate of return. In other words, what you see is what you get without question; an example of such being guaranteed fixed interest rates. In this sense, risk-free rates do not have to be universal or the same across financial instruments such as savings accounts and government bonds, but do have to possess a strongly assured yield. 

Risk-free interest rates do not include a 'risk premium' or an added amount of interest yield that accounts for the risk associated with investing or depositing money into a financial instrument. For example, a 5 year corporate bond from Company A offers 4.5%, whereas a 5 year bond from Company B offers 5.6% These yields differ with perceived risk as measured in part by credit ratings. These ratings are determined by credit rating agency methodologies such as those used by Moody's. 

To illustrate 'risk premium' and 'risk-free rate' further, bond issuers with lower credit ratings have more credit risk associated with them, and are not therefore 'risk free'. The risk premium is determined by market forces such as the rate of return  investors are willing to accept for financial instruments  priced at certain levels with specific levels of risk as defined by credit rating and investor valuation(s). The risk free rate is often bench-marked using a shorter-term financial instrument such as 3-month Treasury Bills according to Rutgers University. 

Even if an issuer has a very high credit rating and is considered 'risk free', that can change. A recent example of this, and as reported by Reuters, was when the Standard and Poor's Credit Rating Agency lowered of the U.S. Government's credit rating from AAA to AA. These changes are somewhat predictable via rating agency 'outlooks', but for longer-term time horizons, are not always so clear. It is for this reason that a risk-free short-term financial instrument is not actually risk free when longer loan terms exist for the same issuer and instrument. 

What constitutes a risk free rate is not constant, and multiple financial vehicles can be considered to have risk-free rates.  For example, a financial statistics class at The Wharton School of Business considers a 1-month Treasury Bill as being risk free instead of a 3-month. This difference in opinion is further highlighted in a report by the financial consulting firm Value Advisor Associates. Moreover, in the report, both 5 year and 10 year financial instruments are considered acceptable proxies for the risk-free rate due  to factors such as upward sloping 'term-structure' i.e. higher rates of return for longer duration bonds.