Index-futures arbitragers only enter into the market if the deviation from the arbitrage relation is sufficiently large to compensate for transaction costs and associated interest rate and dividend risks. We estimate the band around the theoretical futures price within which arbitrage is not profitable for most arbitragers, using a threshold autoregression model. Combining these thresholds with an error-correction model, we show that the impact of the mispricing error is increasing with the magnitude of that error and that the information effect of lagged futures returns on index returns is significantly larger when the mispricing error is negative.