Reference [3] "James, Gareth (2013). An Introduction to Statistical Learning: with Applications in R. Springer. p. 176. ISBN 978-1461471370." is put on the introduction before the table of contents to the parts explaining what a training and validation data set is. Although, when I look into the source, it does not seem to be talking about a validation data set to prevent overfitting (e.g. by "early stopping") but for leave-one-out validations and the like.
Thus, the source does not hold the information it is referencing on the wiki-article.
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