Move reference outliers to a reference outlier sample set (Visual Check)

 Once the clear spectra outliers are remove, we develop the model (In this case N2 in soil) and represent the XY plot (reference values vs. predicted) for the calibration set (blue points) and for the validation set (more than 1000 samples were taken apart from the total set).

Now is our decision to remove some of the samples as reference outliers in the two sets:

Only those that clearly seem reference outliers must be marked, now you can check the statistics, and decide to run the model with other math treatment or calibration strategy (ANN in this case).