training and test sets

If there is no relationship at all between the training set and the test set, then the learning problem is unsolvable; the future can be predicted only if it resembles the past. Therefore, in desiging and studying learning algorithms, we generally assume that the training and test examples are taken from the same random source.

 

< Boosting: Foundations and Algorithms > by Robert E. Shapire and Yoav Freund (2012) pp. 13.