Sometime when validating a product for a certain constituent (in this case dry matter) we can see this type of X-Y plot:
This a not nice at all validation, but we have to see first that we have like to clusters of lab values for lower and higher dry matter. So the first question is:
Which is the range of the calibration samples in the model which I am validating?.
I check and I see that the range for dry matter in the model is from 78,700 to 86,800, so I am validating with samples more dried than the ones in the calibration.
I see that it seems like bias effect for those samples. Let´s remove the samples in range and check the statistics for the samples out of range:
We see that we have a bias effect, and some slope caused but one of the samples. So this is a new source of variation to expand the calibration. Merge the validation samples to the database and recalibrate. Try to make robust the new model for extrapolation.