In the paper "Economic Forces and the Stock Market" by Chen, Roll and Ross, unanticipated risk premium (URP) is tested as a potential risk factor for stock returns. This factor is commonly calculated as the difference between the return on a low-grade bond index and the return on a portfolio of long-term government bonds.
However, in this paper, there is no mention as to what type of return is used to compute URP. Any guess as to whether it would be Month-to-date total return, Yield to maturity, ...?