In k-fold cross-validation, how is the data used?

In k-fold cross-validation, the process involves partitioning the data into k distinct subsets or folds. Each fold takes turns serving as the test set while the remaining k-1 folds are used for training the model. This rotation allows each data point to be used both for training and testing throughout the entire validation process, providing a more comprehensive evaluation of the model's performance.

The key benefit of this approach is that it helps to mitigate any potential issues arising from the random distribution of data by ensuring that each observation is included in both training and testing phases. This enhances the robustness of the model evaluation, as it allows the model's performance to be assessed on multiple different training and testing combinations.

In contrast, using data once would limit the assessment and could lead to either overly optimistic or pessimistic evaluations. Similarly, splitting the data into just two parts would not provide the thoroughness needed for reliable validation across different subsets. Additionally, using data solely for training would neglect the important aspect of testing the model's ability to generalize to new, unseen data.