Fig 1. Procedures for training, testing and validation of the classifiers.

(a) In the first approach the training and test/validation set were treated as totally separate sets. (b) In the second approach batch effects between the training and test set were overcome by surrogate variable analysis, after which the sets were separated and the models were trained and tested. The samples in the validation set were corrected using the surrogate variables twice, labelled as ALS and as control, before assessing the performance of the models.