k-fold validation (k折验证)

本文介绍了如何使用交叉验证来确定KNN模型的最佳K值,通过将数据集分为三折进行训练和验证,最终选择得分最高的K值。此方法在数据量适中时较为有效,但在深度学习和大数据场景下可能过于耗时。

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1、给定若干个模型:比如说具有不同K的KNN(k-Nearest-Neighbor);

2、利用现有的数据,利用交叉验证的方法计算误差,假设如上图所示,分为3折,那么一个KNN就要训练3次,选取3次的平均验证分数作为该KNN的得分;

3、选择得分最高的模型使用的K;

  • 注:通常深度学习中很少用,尤其是当数据量很大的时候,用交叉验证太慢了。
### k-Fold Cross Validation Explained #### Concept of K-Fold Cross Validation K-fold cross-validation is a resampling procedure used to evaluate machine learning models on a limited data sample. The procedure has a single parameter called 'k' that refers to the number of groups into which a given dataset will be split. Each unique group serves once as a test set while the remaining groups form the training set. The main advantage lies in providing more reliable estimates of model performance by averaging over multiple train/test splits, thus reducing variance compared to using a single validation set[^1]. #### Usage and Implementation Example Below demonstrates implementing k-fold cross-validation utilizing Python's `scikit-learn` library: ```python from sklearn.model_selection import KFold import numpy as np # Sample Data Preparation X = np.array([[1, 2], [3, 4], [5, 6], [7, 8]]) y = np.array([1, 0, 1, 0]) kf = KFold(n_splits=2) for train_index, test_index in kf.split(X): X_train, X_test = X[train_index], X[test_index] y_train, y_test = y[train_index], y[test_index] print(f'Training indices:{train_index}, Testing indices:{test_index}') ``` This code snippet initializes a two-fold (`n_splits=2`) cross-validator object named `kf`. For each iteration through the loop, different subsets serve as either training or testing sets based upon index arrays returned during splitting operations performed within `.split()` method calls. Additionally, when evaluating algorithms like those mentioned earlier regarding optimization techniques for handling imbalanced datasets or assessing overall system efficiency across various components, incorporating k-fold strategies ensures comprehensive evaluations without bias towards any particular subset configuration[^2]. --related questions-- 1. How does increasing the value of 'k' affect computational cost? 2. What alternatives exist besides k-fold cross-validation for validating ML models? 3. Can k-fold cross-validation help mitigate issues arising from unbalanced classes? 4. Is there an optimal range recommended for selecting 'k' values? 5. Are certain types of problems better suited for specific numbers of folds?
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