External validation set
The easiest method of the correlation testing is use of an external validation set.  In this method, the correlation is used for predict a property value for a chemical structure that was not used in the creation of the correlation; some test statistics are calculated for the external dataset; the difference between the test statistics in the training and validation datasets is a measure of the reliability of the correlation. The widely used measure is the prediction error sum of squares (PRESS) and is defined as
where ye,i are experimental values of the property and yp,i are predicted values for external validation test.
Often the RMSPE criterion is prefered:
because it gives error on a ‘per compound’ basis. 
The method is a particular case of the leave-many-out cross-validation method.
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