If you had to use a risk premium with the longer periods, what biases will the investor have?

The following table summarizes risk premiums for stocks in the United States, relative to treasury bills and bonds, for different time periods: Risk Premium for Equity 1928-2011 1962-2011 2002-2011 Stocks - T.Bills Arithmetic 7.55% 5.38% 3.12% Stocks - T.Bonds Geometric Arithmetic Geometric 5.79% 5.62% 4.10% 3.36% 4.02% 2.35% -1.92% 1.08% -3.61%