Cross validation | DRY + IRR | DRY | IRR |
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\({\text{R}}_{\text{CV}}^{2}\) | RMSECV | RMSEC | RMSECV/RMSEC | F | | \({\text{R}}_{\text{CV}}^{2}\) | RMSECV | RMSEC | RMSECV/RMSEC | F | | \({\text{R}}_{\text{CV}}^{2}\) | RMSECV | RMSEC | RMSECV/RMSEC | F | |
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No preprocessing | 0.745 | 1.541 | 1.522 | 1.013 | 7 | | 0.777 | 1.367 | 1.333 | 1.026 | 7 | | 0.682 | 1.566 | 1.535 | 1.020 | 7 | |

Smoothing | 0.746 | 1.540 | 1.522 | 1.012 | 7 | | 0.778 | 1.365 | 1.333 | 1.024 | 7 | | 0.680 | 1.571 | 1.535 | 1.023 | 7 | |

Smoothing + normalization | 0.740 | 1.556 | 1.537 | 1.012 | 6 | | 0.779 | 1.361 | 1.334 | 1.020 | 6 | | 0.686 | 1.556 | 1.521 | 1.023 | 7 | |

Smoothing + baseline + normalization | 0.738 | 1.564 | 1.550 | 1.009 | 7 | | 0.776 | 1.369 | 1.341 | 1.021 | 7 | | 0.694 | 1.534 | 1.502 | 1.022 | 7 | |

SNV | 0.746 | 1.539 | 1.526 | 1.009 | 6 | | 0.780 | 1.359 | 1.331 | 1.021 | 6 | | 0.686 | 1.555 | 1.522 | 1.021 | 6 | |

SNV + DT | 0.747 | 1.536 | 1.517 | 1.013 | 7 | | 0.788 | 1.334 | 1.295 | 1.030 | 7 | | 0.685 | 1.558 | 1.523 | 1.023 | 6 | |

SNV + DT + 1st derivative | 0.746 | 1.539 | 1.518 | 1.014 | 5 | | 0.776 | 1.371 | 1.355 | 1.012 | 3 | | 0.691 | 1.543 | 1.528 | 1.009 | 3 | |

SNV + DT + 2nd derivative | 0.729 | 1.590 | 1.569 | 1.013 | 3 | | 0.820 | 1.230 | 1.070 | 1.149 | 7 | | 0.688 | 1.551 | 1.510 | 1.027 | 3 | |

External validation | \({\text{R}}_{\text{P}}^{2}\) | RMSEP | RMSEC | RMSEP/RMSEC | F | RPD | \({\text{R}}_{\text{P}}^{2}\) | RMSEP | RMSEC | RMSEP/RMSEC | F | RPD | \({\text{R}}_{\text{P}}^{2}\) | RMSEP | RMSEC | RMSEP/RMSEC | F | RPD |
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SNV + DT + 2nd derivative | 0.764 | 1.486 | 1.437 | 1.034 | 6 | 2.056 | 0.829 | 1.200 | 1.070 | 1.121 | 7 | 2.413 | 0.765 | 1.343 | 1.297 | 1.036 | 7 | 2.065 |

- Smoothing: median filter smoothing; Normalization: normalization by mean; Baseline: offset and linear baseline correction; SNV: standard normal variate; DT: de-trending, 2nd polynomial; Savitzky–Golay 1st derivative, 2nd polynomial, 23 smoothing points; Savitzky–Golay 2nd derivative, 2nd polynomial, 23 smoothing points
- R
^{2}, coefficient of determination; RMSECV, root mean squared error of cross validation; RMSEC, root mean squares error of calibration; RMSEP, root mean squared error of prediction; CV, cross validation; EV, external validation; F, number of factors (latent values) used in calibration; RPD, ratio of performance over prediction