External validation set

The easiest method of the correlation testing is use of an external validation set. [1] 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 y_e,i are experimental values of the property and   y_p,i  are predicted values for external validation test.

Often the RMSPE criterion is prefered:

because it gives error  on a ‘per compound’ basis. [1]

The method is a particular case of the leave-many-out cross-validation method.

References:

1. Stone M.; Jonathan P. Statistical Thinking and Technique for QSAR and related studies. 1. Genaral Theory J. Chemometrics 1993, 7, 455-475.