I encountered a question where I have to use the Ibbotson-Chen equation, and I was given the long term government bond yield and short term government bond yield. I am struggled to understand why the answer says we should use the long-term yield and not the short-term yield as expected risk free rate 
Equation: [(1+expected inflation)(1+real earning growth)(1+P/E growth)-1]+ Income growth - Expected risk free rate
I always thought risk free rate is assumed to be short-term government issued securities…I am confused, am I correct? If so when do I know when to use short term or long term… 