Other rationales for the failure of the forward rate unbiasedness hypothesis include considering the conditional bias to be an exogenous variable explained by a policy aimed at smoothing interest rates and stabilizing exchange rates, or considering that an economy allowing for discrete changes could facilitate excess returns in the forward market. Some researchers have contested empirical failures of the hypothesis and have sought to explain conflicting evidence as resulting from contaminated data and even inappropriate selections of the time length of forward contracts. Economists demonstrated that the forward rate could serve as a useful proxy for future spot exchange rates between currencies with liquidity premia that average out to zero during the onset of floating exchange rate regimes in the 1970s. Research examining the introduction of endogenous breaks to test the structural stability of cointegrated spot and forward exchange rate time series have found some evidence to support forward rate unbiasedness in both the short and long term.