In a k-fold cross-validation, we divide the data into k groups. Are these groups called the "folds"? Can the word "fold" be used when I talk about these groups? Is it usual? Is the "fold" even a noun in this context in the statistical English?

If not, how do you call these k groups? Just "groups"?

PS: I am not a native speaker so I cannot judge it by "feeling"... I haven't found any dictionary definition of the noun "fold" that would be fitting. Anyway, I do not feel like asking this question other people than statisticians, since the English language used in statistics might be very different from the common English.


1 Answer 1


Yes, you can use fold as a noun when referring to the groups:

An alternative to LOOCV is k-fold CV. This approach involves randomly k-fold CV dividing the set of observations into k groups, or folds, of approximately equal size. The first fold is treated as a validation set...

Source: Chapter 5.1.3 in Introduction to Statistical Learning by James, Witten, Hastie and Tibshirani.

  • $\begingroup$ Thank you!! Looks like the main author, Gareth James, is a native speaker as he is from New Zealand, and the other authors are native speakers too :-)) $\endgroup$
    – Tomas
    Jul 7, 2021 at 15:10
  • $\begingroup$ You are totally right. Another proof :-)) blockCV: An r package for generating spatially or environmentally separated folds for k-fold cross-validation of species distribution models besjournals.onlinelibrary.wiley.com/doi/10.1111/2041-210X.13107 $\endgroup$
    – Tomas
    Jul 7, 2021 at 15:20
  • 2
    $\begingroup$ I am not a native speaker, but I am using the word myself and I have heard it multiple times from many statisticians. When teaching, I use both "folds" and "groups" and I would not say that I prefer one to another. ISLR is just an example that I had at hand, but I definitely encountered "fold" in other textooks. So yes, I would say it is a common term. $\endgroup$
    – Misius
    Jul 7, 2021 at 16:02
  • $\begingroup$ Thanks @Misius! You are using the word "fold" in which context? Cross-validation? $\endgroup$
    – Tomas
    Jul 7, 2021 at 19:54
  • 1
    $\begingroup$ @Tomas Yes, exactly in the context you described. $\endgroup$
    – Misius
    Jul 7, 2021 at 20:13

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