It has been found that the expectation hypothesis has been tested and rejected using a wide variety of interest rates, over a variety of time periods and monetary policy regimes.[2] This analysis is supported in a study conducted by Sarno,[3] where it is concluded that while conventional bivariate procedure provides mixed results, the more powerful testing procedures, for example expanded vector autoregression test, suggest rejection of the expectation hypothesis throughout the maturity spectrum examined. A common reason given for the failure of the expectation hypothesis is that the risk premium is not constant as the expectation hypothesis requires, but is time-varying. However, research by Guidolin and Thornton (2008) suggest otherwise.[2] It is postulated that the expectation hypothesis fails because short-term interest rates are not predictable to any significant degree.