1. Method for measuring characteristics of a sample by spectral analysis, comprising: calculating the characteristics from spectral data obtained from the spectral analysis using a calibration model that has been established on the basis of reference samples; performing an additional calculation of the characteristics of the sample based on the same spectral data using at least one additional calibration model that has been established on the basis of additional reference samples; and determining and outputting deviations between the characteristics calculated by the respective calibration models, to permit an evaluation of the quality of the measurement.