A computer-implemented method for determining the refractive error of an eye of a wearer of spectacles, comprising: providing measured values from a first and a second measurement of the refractive error of the eye of the spectacle wearer; calculating an estimate of the refractive error of the eye of the eyeglass wearer from the measurements from the first and second measurements, the calculation of the estimate of the refractive error taking into account measurement inaccuracies of the first and second refractive error measurements, the measurement inaccuracies comprising a systematic deviation between the measurements from the first measurement and the measurements from the second measurement; and the method further comprising determining the measurement inaccuracies of the first and second measurements by statistical analysis of a data set comprising a plurality of reference measurements including a first measurement and a second measurement of the refractive error of the eyes of different spectacle wearers, wherein determining the measurement inaccuracies of the first measurement and the second measurements comprises: establishing a model for the measurement values of the second measurement as a sum of a predicted measurement value and a random variable, wherein the predicted measurement value is modelled as a parametric function of the measurement value of the first measurement and optionally a portion of the measurement value of the second measurement; determining the parameters of the parametric function by fitting the model to the reference measurements in the data set while maximising the probability distribution of the random variables in the parameter space of the model; determining a systematic deviation of the first measurement from the second measurement using the predicted measurement. providing measured values from a first and a second measurement of the refractive error of the eye of the spectacle wearer; calculating an estimate of the refractive error of the eye of the eyeglass wearer from the measurements from the first and second measurements, the calculation of the estimate of the refractive error taking into account measurement inaccuracies of the first and second refractive error measurements, the measurement inaccuracies comprising a systematic deviation between the measurements from the first measurement and the measurements from the second measurement; and the method further comprising determining the measurement inaccuracies of the first and second measurements by statistical analysis of a data set comprising a plurality of reference measurements including a first measurement and a second measurement of the refractive error of the eyes of different spectacle wearers, wherein determining the measurement inaccuracies of the first measurement and the second measurements comprises: establishing a model for the measurement values of the second measurement as a sum of a predicted measurement value and a random variable, wherein the predicted measurement value is modelled as a parametric function of the measurement value of the first measurement and optionally a portion of the measurement value of the second measurement; determining the parameters of the parametric function by fitting the model to the reference measurements in the data set while maximising the probability distribution of the random variables in the parameter space of the model; determining a systematic deviation of the first measurement from the second measurement using the predicted measurement. establishing a model for the measurement values of the second measurement as a sum of a predicted measurement value and a random variable, wherein the predicted measurement value is modelled as a parametric function of the measurement value of the first measurement and optionally a portion of the measurement value of the second measurement; determining the parameters of the parametric function by fitting the model to the reference measurements in the data set while maximising the probability distribution of the random variables in the parameter space of the model; determining a systematic deviation of the first measurement from the second measurement using the predicted measurement.