What do investors sometimes use as a proxy for the risk-free rate?

6) What do investors sometimes use as a proxy for the risk-free rate? 7) How would you define the market risk premium? 8) Given the following historical returns, what was the historical risk premium of Corporate bonds?, what was the historical risk premium for Small stocks?, What was the historical risk premium for the "Market"? Corporate bonds: 6.6% Inflation: 3% S&P 500: 11.5% Treasury Bills: 4.5% Treasury bonds: 5.5% Small Cap Stocks: 13% 9) Why might the risk free estimate we use today be different than the historical risk-free rate