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Train and Compare Classifiers Using Misclassification Costs in Classification Learner App

This example shows how to create and compare classifiers that use specified misclassification costs in the Classification Learner app. Specify the misclassification costs before training, and use the accuracy and total misclassification cost results to compare the trained models.

  1. In the MATLAB® Command Window, read the sample file CreditRating_Historical.dat into a table. The predictor data consists of financial ratios and industry sector information for a list of corporate customers. The response variable consists of credit ratings assigned by a rating agency. Combine all the A ratings into one rating. Do the same for the B and C ratings, so that the response variable has three distinct ratings. Among the three ratings, A is considered the best and C the worst.

    creditrating = readtable('CreditRating_Historical.dat');
    Rating = categorical(creditrating.Rating);
    Rating = mergecats(Rating,{'AAA','AA','A'},'A');
    Rating = mergecats(Rating,{'BBB','BB','B'},'B');
    Rating = mergecats(Rating,{'CCC','CC','C'},'C');
    creditrating.Rating = Rating;
  2. Assume these are the costs associated with misclassifying the credit ratings of customers.

     Customer Predicted Rating
    ABC
    Customer True RatingA$0$100$200
    B$500$0$100
    C$1000$500$0

    For example, the cost of misclassifying a C rating customer as an A rating customer is $1000. The costs indicate that classifying a customer with bad credit as a customer with good credit is more costly than classifying a customer with good credit as a customer with bad credit.

    Create a matrix variable that contains the misclassification costs. Create another variable that specifies the class names and their order in the matrix variable.

    ClassificationCosts = [0 100 200; 500 0 100; 1000 500 0];
    ClassNames = categorical({'A','B','C'});
    

    Tip

    Alternatively, you can specify misclassification costs directly inside the Classification Learner app. See Specify Misclassification Costs for more information.

  3. Open Classification Learner. Click the Apps tab, and then click the arrow at the right of the Apps section to open the apps gallery. In the Machine Learning and Deep Learning group, click Classification Learner.

    Classification Learner tab

  4. On the Classification Learner tab, in the File section, select New Session > From Workspace.

  5. In the New Session from Workspace dialog box, select the table creditrating from the Data Set Variable list.

    As shown in the dialog box, the app selects the response and predictor variables based on their data type. The default response variable is the Rating variable. The default validation option is cross-validation, to protect against overfitting. For this example, do not change the default settings.

    New Session from Workspace dialog box

  6. To accept the default settings, click Start Session.

  7. Specify the misclassification costs. On the Classification Learner tab, in the Options section, click Misclassification Costs. The app opens a dialog box showing the default misclassification costs.

    In the dialog box, click Import from Workspace.

    Specify misclassification costs dialog box. By default, correct classifications have a cost of 0, and incorrect classifications have a cost of 1.

    In the import dialog box, select ClassificationCosts as the cost variable and ClassNames as the class order in the cost variable. Click Import.

    Import costs from workspace dialog box

    The app updates the values in the misclassification costs dialog box. Click OK to save your changes.

    Dialog box with updated values for misclassification costs

  8. Train fine, medium, and coarse trees simultaneously. On the Classification Learner tab, in the Model Type section, click the arrow to open the gallery. In the Decision Trees group, click All Trees. In the Training section, click Train. The app trains one of each tree model type and displays the models in the Models pane.

    Tip

    If you have Parallel Computing Toolbox™, you can train all the models (All Trees) simultaneously by selecting the Use Parallel button in the Training section before clicking Train. After you click Train, the Opening Parallel Pool dialog box opens and remains open while the app opens a parallel pool of workers. During this time, you cannot interact with the software. After the pool opens, the app trains the models simultaneously.

    Validation confusion matrix for the fine tree model. Blue values indicate correct classifications, and red values indicate incorrect classifications.

    Note

    Validation introduces some randomness into the results. Your model validation results can vary from the results shown in this example.

  9. In the Models pane, click a model to view the results, which are displayed in the Current Model Summary pane. Each model has a validation accuracy score that indicates the percentage of correctly predicted responses. In the Models pane, the app highlights the highest Accuracy (Validation) score by outlining it in a box.

  10. Inspect the accuracy of the predictions in each class. On the Classification Learner tab, in the Plots section, click the arrow to open the gallery, and then click Confusion Matrix (Validation) in the Validation Results group. The app displays a matrix of true class and predicted class results for the selected model (in this case, for the medium tree).

    Validation confusion matrix for the medium tree model. Blue values indicate correct classifications, and red values indicate incorrect classifications.

  11. You can also plot results per predicted class to investigate false discovery rates. Under Plot, select the Positive Predictive Values (PPV) False Discovery Rates (FDR) option.

    In the confusion matrix for the medium tree, the entries below the diagonal have small percentage values. These values indicate that the model tries to avoid assigning a credit rating that is higher than the true rating for a customer.

    Validation confusion matrix, displayed with positive predictive values and false discovery rates

  12. Compare the total misclassification costs of the tree models. To inspect the total misclassification cost of a model, select the model in the Models pane, and then view the Training Results section of the Current Model Summary pane. For example, the medium tree has these results.

    Current Model Summary pane with results for the medium tree

    Alternatively, you can sort the models based on the total misclassification cost. In the Models pane, open the Sort by list and select Total Cost (Validation).

    In general, choose a model that has high accuracy and low total misclassification cost. In this example, the medium tree has the highest validation accuracy value and the lowest total misclassification cost of the three models.

You can perform feature selection and transformation or tune your model just as you do in the workflow without misclassification costs. However, always check the total misclassification cost of your model when assessing its performance. For differences in the exported model and exported code when you use misclassification costs, see Misclassification Costs in Exported Model and Generated Code.

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