When using k-fold cross-validation with k=5, how many times is each observation used for validation?
Select an answer to reveal the explanation.
Short Explanation
Here's the deal — in five-fold cross-validation, each observation sits in exactly one fold, so it's held out for validation just once. You'll see it in training four times, but validation only once.
Full Explanation
Exactly 1 time, in the single fold where it is held out, is correct because each observation belongs to exactly one fold and is validated only when that fold is held out, which happens once. The five-times option is wrong because validation occurs only once. The option saying four times in every fold except the one it belongs to is wrong because that counts training-set appearances. The option saying four times as it is in the training set for four folds is also wrong for the same reason.