I'm working on an assignment in which I need to calculate excess return for six stocks plus the S&P 500. I have computed daily logarithmic returns for every stock and for the market, I now need to calculate the risk free interest rate in order to be able to compute the excess return for every stock and the market.

Interest rates on three months T-Bills are a good proxy for the risk-free rate of return, but I have a lot of doubts on how to use data provided by Yahoo! Finance in order to compute the daily risk-free. Here are my assumptions and procedures:

  • I use the 13 week treasury bill (ticker: ^IRX) historical quotes provided by Yahoo! Finance;
  • Under the assumption that on Yahoo! Finance bonds yields are quoted as Effective Annual Rate (EAR), daily risk-free interest rate at time $t$ ($r_{f,t}^{daily}$) is computed as:


where $r_t$ is the EAR rate at time $t$ provided by Yahoo.

Once computations are done, excess return for stock $i$ at time $t$ is defined as:

$$\text{Excess Return}=r_{i,t}-r_{f,t}^{daily}$$


  • Is this procedure correct? Note that I do not need it to be exceptionally precise, it is just a basic exercise, but I would like it to be at least conceptually correct.
  • The Federal Reserve also provide data for three months T-Bills. Are this rates also provided as EAR?
  • $\begingroup$ I have the same question. Have you found out the question? Could you share? Thanks. $\endgroup$ – user233051 Mar 17 '17 at 7:54
  • $\begingroup$ @user233051 : I have answered your question and his. See your posed question here. $\endgroup$ – Mike O'Connor Aug 12 '18 at 8:33

Just use the what most finance research papers use, i.e. the risk-free rate from the Kenneth French data library.


The rates are annual. So if you want log returns just take the log of $1+r^f_t$ and divide by 365.

  • $\begingroup$ I'm not sure if the rates are annual. For monthly, you have monthly rates, for daily, the same daily rates $\endgroup$ – samantha Apr 21 '18 at 13:06

I think we can find daily risk free using following equation $$1 + r_{90} = (1 + r_1)^{90}$$ This follows from the fact there are no arbitrage opportunity. Here we assume that $r_1$ in the following periods will stay the same, that is non random. Thus, doing simple algebra we get $$r_1 = (1+r_{90})^{\frac{1}{90}} - 1$$


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