.. DO NOT EDIT. .. THIS FILE WAS AUTOMATICALLY GENERATED BY SPHINX-GALLERY. .. TO MAKE CHANGES, EDIT THE SOURCE PYTHON FILE: .. "auto_examples/model_selection/plot_cost_sensitive_learning.py" .. LINE NUMBERS ARE GIVEN BELOW. .. only:: html .. note:: :class: sphx-glr-download-link-note :ref:`Go to the end ` to download the full example code. or to run this example in your browser via JupyterLite or Binder .. rst-class:: sphx-glr-example-title .. _sphx_glr_auto_examples_model_selection_plot_cost_sensitive_learning.py: ============================================================== Post-tuning the decision threshold for cost-sensitive learning ============================================================== Once a classifier is trained, the output of the :term:`predict` method outputs class label predictions corresponding to a thresholding of either the :term:`decision_function` or the :term:`predict_proba` output. For a binary classifier, the default threshold is defined as a posterior probability estimate of 0.5 or a decision score of 0.0. However, this default strategy is most likely not optimal for the task at hand. Here, we use the "Statlog" German credit dataset [1]_ to illustrate a use case. In this dataset, the task is to predict whether a person has a "good" or "bad" credit. In addition, a cost-matrix is provided that specifies the cost of misclassification. Specifically, misclassifying a "bad" credit as "good" is five times more costly on average than misclassifying a "good" credit as "bad". We use the :class:`~sklearn.model_selection.TunedThresholdClassifierCV` to select the cut-off point of the decision function that minimizes the provided business cost. In the second part of the example, we further extend this approach by considering the problem of fraud detection in credit card transactions: in this case, the business metric depends on the amount of each individual transaction. .. rubric :: References .. [1] "Statlog (German Credit Data) Data Set", UCI Machine Learning Repository, `Link `_. .. [2] `Charles Elkan, "The Foundations of Cost-Sensitive Learning", International joint conference on artificial intelligence. Vol. 17. No. 1. Lawrence Erlbaum Associates Ltd, 2001. `_ .. GENERATED FROM PYTHON SOURCE LINES 37-41 .. code-block:: Python # Authors: The scikit-learn developers # SPDX-License-Identifier: BSD-3-Clause .. GENERATED FROM PYTHON SOURCE LINES 42-55 Cost-sensitive learning with constant gains and costs ----------------------------------------------------- In this first section, we illustrate the use of the :class:`~sklearn.model_selection.TunedThresholdClassifierCV` in a setting of cost-sensitive learning when the gains and costs associated to each entry of the confusion matrix are constant. We use the problematic presented in [2]_ using the "Statlog" German credit dataset [1]_. "Statlog" German credit dataset ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ We fetch the German credit dataset from OpenML. .. GENERATED FROM PYTHON SOURCE LINES 55-63 .. code-block:: Python import sklearn from sklearn.datasets import fetch_openml sklearn.set_config(transform_output="pandas") german_credit = fetch_openml(data_id=31, as_frame=True, parser="pandas") X, y = german_credit.data, german_credit.target .. GENERATED FROM PYTHON SOURCE LINES 64-65 We check the feature types available in `X`. .. GENERATED FROM PYTHON SOURCE LINES 65-67 .. code-block:: Python X.info() .. rst-class:: sphx-glr-script-out .. code-block:: none RangeIndex: 1000 entries, 0 to 999 Data columns (total 20 columns): # Column Non-Null Count Dtype --- ------ -------------- ----- 0 checking_status 1000 non-null category 1 duration 1000 non-null int64 2 credit_history 1000 non-null category 3 purpose 1000 non-null category 4 credit_amount 1000 non-null int64 5 savings_status 1000 non-null category 6 employment 1000 non-null category 7 installment_commitment 1000 non-null int64 8 personal_status 1000 non-null category 9 other_parties 1000 non-null category 10 residence_since 1000 non-null int64 11 property_magnitude 1000 non-null category 12 age 1000 non-null int64 13 other_payment_plans 1000 non-null category 14 housing 1000 non-null category 15 existing_credits 1000 non-null int64 16 job 1000 non-null category 17 num_dependents 1000 non-null int64 18 own_telephone 1000 non-null category 19 foreign_worker 1000 non-null category dtypes: category(13), int64(7) memory usage: 69.9 KB .. GENERATED FROM PYTHON SOURCE LINES 68-70 Many features are categorical and usually string-encoded. We need to encode these categories when we develop our predictive model. Let's check the targets. .. GENERATED FROM PYTHON SOURCE LINES 70-72 .. code-block:: Python y.value_counts() .. rst-class:: sphx-glr-script-out .. code-block:: none class good 700 bad 300 Name: count, dtype: int64 .. GENERATED FROM PYTHON SOURCE LINES 73-81 Another observation is that the dataset is imbalanced. We would need to be careful when evaluating our predictive model and use a family of metrics that are adapted to this setting. In addition, we observe that the target is string-encoded. Some metrics (e.g. precision and recall) require to provide the label of interest also called the "positive label". Here, we define that our goal is to predict whether or not a sample is a "bad" credit. .. GENERATED FROM PYTHON SOURCE LINES 81-83 .. code-block:: Python pos_label, neg_label = "bad", "good" .. GENERATED FROM PYTHON SOURCE LINES 84-85 To carry our analysis, we split our dataset using a single stratified split. .. GENERATED FROM PYTHON SOURCE LINES 85-89 .. code-block:: Python from sklearn.model_selection import train_test_split X_train, X_test, y_train, y_test = train_test_split(X, y, stratify=y, random_state=0) .. GENERATED FROM PYTHON SOURCE LINES 90-105 We are ready to design our predictive model and the associated evaluation strategy. Evaluation metrics ^^^^^^^^^^^^^^^^^^ In this section, we define a set of metrics that we use later. To see the effect of tuning the cut-off point, we evaluate the predictive model using the Receiver Operating Characteristic (ROC) curve and the Precision-Recall curve. The values reported on these plots are therefore the true positive rate (TPR), also known as the recall or the sensitivity, and the false positive rate (FPR), also known as the specificity, for the ROC curve and the precision and recall for the Precision-Recall curve. From these four metrics, scikit-learn does not provide a scorer for the FPR. We therefore need to define a small custom function to compute it. .. GENERATED FROM PYTHON SOURCE LINES 105-115 .. code-block:: Python from sklearn.metrics import confusion_matrix def fpr_score(y, y_pred, neg_label, pos_label): cm = confusion_matrix(y, y_pred, labels=[neg_label, pos_label]) tn, fp, _, _ = cm.ravel() tnr = tn / (tn + fp) return 1 - tnr .. GENERATED FROM PYTHON SOURCE LINES 116-124 As previously stated, the "positive label" is not defined as the value "1" and calling some of the metrics with this non-standard value raise an error. We need to provide the indication of the "positive label" to the metrics. We therefore need to define a scikit-learn scorer using :func:`~sklearn.metrics.make_scorer` where the information is passed. We store all the custom scorers in a dictionary. To use them, we need to pass the fitted model, the data and the target on which we want to evaluate the predictive model. .. GENERATED FROM PYTHON SOURCE LINES 124-134 .. code-block:: Python from sklearn.metrics import make_scorer, precision_score, recall_score tpr_score = recall_score # TPR and recall are the same metric scoring = { "precision": make_scorer(precision_score, pos_label=pos_label), "recall": make_scorer(recall_score, pos_label=pos_label), "fpr": make_scorer(fpr_score, neg_label=neg_label, pos_label=pos_label), "tpr": make_scorer(tpr_score, pos_label=pos_label), } .. GENERATED FROM PYTHON SOURCE LINES 135-149 In addition, the original research [1]_ defines a custom business metric. We call a "business metric" any metric function that aims at quantifying how the predictions (correct or wrong) might impact the business value of deploying a given machine learning model in a specific application context. For our credit prediction task, the authors provide a custom cost-matrix which encodes that classifying a a "bad" credit as "good" is 5 times more costly on average than the opposite: it is less costly for the financing institution to not grant a credit to a potential customer that will not default (and therefore miss a good customer that would have otherwise both reimbursed the credit and payed interests) than to grant a credit to a customer that will default. We define a python function that weight the confusion matrix and return the overall cost. .. GENERATED FROM PYTHON SOURCE LINES 149-181 .. code-block:: Python import numpy as np def credit_gain_score(y, y_pred, neg_label, pos_label): cm = confusion_matrix(y, y_pred, labels=[neg_label, pos_label]) # The rows of the confusion matrix hold the counts of observed classes # while the columns hold counts of predicted classes. Recall that here we # consider "bad" as the positive class (second row and column). # Scikit-learn model selection tools expect that we follow a convention # that "higher" means "better", hence the following gain matrix assigns # negative gains (costs) to the two kinds of prediction errors: # - a gain of -1 for each false positive ("good" credit labeled as "bad"), # - a gain of -5 for each false negative ("bad" credit labeled as "good"), # The true positives and true negatives are assigned null gains in this # metric. # # Note that theoretically, given that our model is calibrated and our data # set representative and large enough, we do not need to tune the # threshold, but can safely set it to the cost ration 1/5, as stated by Eq. # (2) in Elkan paper [2]_. gain_matrix = np.array( [ [0, -1], # -1 gain for false positives [-5, 0], # -5 gain for false negatives ] ) return np.sum(cm * gain_matrix) scoring["credit_gain"] = make_scorer( credit_gain_score, neg_label=neg_label, pos_label=pos_label ) .. GENERATED FROM PYTHON SOURCE LINES 182-187 Vanilla predictive model ^^^^^^^^^^^^^^^^^^^^^^^^ We use :class:`~sklearn.ensemble.HistGradientBoostingClassifier` as a predictive model that natively handles categorical features and missing values. .. GENERATED FROM PYTHON SOURCE LINES 187-194 .. code-block:: Python from sklearn.ensemble import HistGradientBoostingClassifier model = HistGradientBoostingClassifier( categorical_features="from_dtype", random_state=0 ).fit(X_train, y_train) model .. raw:: html

HistGradientBoostingClassifier(random_state=0)

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.. GENERATED FROM PYTHON SOURCE LINES 195-197 We evaluate the performance of our predictive model using the ROC and Precision-Recall curves. .. GENERATED FROM PYTHON SOURCE LINES 197-238 .. code-block:: Python import matplotlib.pyplot as plt from sklearn.metrics import PrecisionRecallDisplay, RocCurveDisplay fig, axs = plt.subplots(nrows=1, ncols=2, figsize=(14, 6)) PrecisionRecallDisplay.from_estimator( model, X_test, y_test, pos_label=pos_label, ax=axs[0], name="GBDT" ) axs[0].plot( scoring["recall"](model, X_test, y_test), scoring["precision"](model, X_test, y_test), marker="o", markersize=10, color="tab:blue", label="Default cut-off point at a probability of 0.5", ) axs[0].set_title("Precision-Recall curve") axs[0].legend() RocCurveDisplay.from_estimator( model, X_test, y_test, pos_label=pos_label, ax=axs[1], name="GBDT", plot_chance_level=True, ) axs[1].plot( scoring["fpr"](model, X_test, y_test), scoring["tpr"](model, X_test, y_test), marker="o", markersize=10, color="tab:blue", label="Default cut-off point at a probability of 0.5", ) axs[1].set_title("ROC curve") axs[1].legend() _ = fig.suptitle("Evaluation of the vanilla GBDT model") .. image-sg:: /auto_examples/model_selection/images/sphx_glr_plot_cost_sensitive_learning_001.png :alt: Evaluation of the vanilla GBDT model, Precision-Recall curve, ROC curve :srcset: /auto_examples/model_selection/images/sphx_glr_plot_cost_sensitive_learning_001.png :class: sphx-glr-single-img .. GENERATED FROM PYTHON SOURCE LINES 239-253 We recall that these curves give insights on the statistical performance of the predictive model for different cut-off points. For the Precision-Recall curve, the reported metrics are the precision and recall and for the ROC curve, the reported metrics are the TPR (same as recall) and FPR. Here, the different cut-off points correspond to different levels of posterior probability estimates ranging between 0 and 1. By default, `model.predict` uses a cut-off point at a probability estimate of 0.5. The metrics for such a cut-off point are reported with the blue dot on the curves: it corresponds to the statistical performance of the model when using `model.predict`. However, we recall that the original aim was to minimize the cost (or maximize the gain) as defined by the business metric. We can compute the value of the business metric: .. GENERATED FROM PYTHON SOURCE LINES 253-255 .. code-block:: Python print(f"Business defined metric: {scoring['credit_gain'](model, X_test, y_test)}") .. rst-class:: sphx-glr-script-out .. code-block:: none Business defined metric: -232 .. GENERATED FROM PYTHON SOURCE LINES 256-274 At this stage we don't know if any other cut-off can lead to a greater gain. To find the optimal one, we need to compute the cost-gain using the business metric for all possible cut-off points and choose the best. This strategy can be quite tedious to implement by hand, but the :class:`~sklearn.model_selection.TunedThresholdClassifierCV` class is here to help us. It automatically computes the cost-gain for all possible cut-off points and optimizes for the `scoring`. .. _cost_sensitive_learning_example: Tuning the cut-off point ^^^^^^^^^^^^^^^^^^^^^^^^ We use :class:`~sklearn.model_selection.TunedThresholdClassifierCV` to tune the cut-off point. We need to provide the business metric to optimize as well as the positive label. Internally, the optimum cut-off point is chosen such that it maximizes the business metric via cross-validation. By default a 5-fold stratified cross-validation is used. .. GENERATED FROM PYTHON SOURCE LINES 274-284 .. code-block:: Python from sklearn.model_selection import TunedThresholdClassifierCV tuned_model = TunedThresholdClassifierCV( estimator=model, scoring=scoring["credit_gain"], store_cv_results=True, # necessary to inspect all results ) tuned_model.fit(X_train, y_train) print(f"{tuned_model.best_threshold_=:0.2f}") .. rst-class:: sphx-glr-script-out .. code-block:: none tuned_model.best_threshold_=0.02 .. GENERATED FROM PYTHON SOURCE LINES 285-288 We plot the ROC and Precision-Recall curves for the vanilla model and the tuned model. Also we plot the cut-off points that would be used by each model. Because, we are reusing the same code later, we define a function that generates the plots. .. GENERATED FROM PYTHON SOURCE LINES 288-364 .. code-block:: Python def plot_roc_pr_curves(vanilla_model, tuned_model, *, title): fig, axs = plt.subplots(nrows=1, ncols=3, figsize=(21, 6)) linestyles = ("dashed", "dotted") markerstyles = ("o", ">") colors = ("tab:blue", "tab:orange") names = ("Vanilla GBDT", "Tuned GBDT") for idx, (est, linestyle, marker, color, name) in enumerate( zip((vanilla_model, tuned_model), linestyles, markerstyles, colors, names) ): decision_threshold = getattr(est, "best_threshold_", 0.5) PrecisionRecallDisplay.from_estimator( est, X_test, y_test, pos_label=pos_label, linestyle=linestyle, color=color, ax=axs[0], name=name, ) axs[0].plot( scoring["recall"](est, X_test, y_test), scoring["precision"](est, X_test, y_test), marker, markersize=10, color=color, label=f"Cut-off point at probability of {decision_threshold:.2f}", ) RocCurveDisplay.from_estimator( est, X_test, y_test, pos_label=pos_label, linestyle=linestyle, color=color, ax=axs[1], name=name, plot_chance_level=idx == 1, ) axs[1].plot( scoring["fpr"](est, X_test, y_test), scoring["tpr"](est, X_test, y_test), marker, markersize=10, color=color, label=f"Cut-off point at probability of {decision_threshold:.2f}", ) axs[0].set_title("Precision-Recall curve") axs[0].legend() axs[1].set_title("ROC curve") axs[1].legend() axs[2].plot( tuned_model.cv_results_["thresholds"], tuned_model.cv_results_["scores"], color="tab:orange", ) axs[2].plot( tuned_model.best_threshold_, tuned_model.best_score_, "o", markersize=10, color="tab:orange", label="Optimal cut-off point for the business metric", ) axs[2].legend() axs[2].set_xlabel("Decision threshold (probability)") axs[2].set_ylabel("Objective score (using cost-matrix)") axs[2].set_title("Objective score as a function of the decision threshold") fig.suptitle(title) .. GENERATED FROM PYTHON SOURCE LINES 365-368 .. code-block:: Python title = "Comparison of the cut-off point for the vanilla and tuned GBDT model" plot_roc_pr_curves(model, tuned_model, title=title) .. image-sg:: /auto_examples/model_selection/images/sphx_glr_plot_cost_sensitive_learning_002.png :alt: Comparison of the cut-off point for the vanilla and tuned GBDT model, Precision-Recall curve, ROC curve, Objective score as a function of the decision threshold :srcset: /auto_examples/model_selection/images/sphx_glr_plot_cost_sensitive_learning_002.png :class: sphx-glr-single-img .. GENERATED FROM PYTHON SOURCE LINES 369-385 The first remark is that both classifiers have exactly the same ROC and Precision-Recall curves. It is expected because by default, the classifier is fitted on the same training data. In a later section, we discuss more in detail the available options regarding model refitting and cross-validation. The second remark is that the cut-off points of the vanilla and tuned model are different. To understand why the tuned model has chosen this cut-off point, we can look at the right-hand side plot that plots the objective score that is our exactly the same as our business metric. We see that the optimum threshold corresponds to the maximum of the objective score. This maximum is reached for a decision threshold much lower than 0.5: the tuned model enjoys a much higher recall at the cost of of significantly lower precision: the tuned model is much more eager to predict the "bad" class label to larger fraction of individuals. We can now check if choosing this cut-off point leads to a better score on the testing set: .. GENERATED FROM PYTHON SOURCE LINES 385-387 .. code-block:: Python print(f"Business defined metric: {scoring['credit_gain'](tuned_model, X_test, y_test)}") .. rst-class:: sphx-glr-script-out .. code-block:: none Business defined metric: -134 .. GENERATED FROM PYTHON SOURCE LINES 388-407 We observe that tuning the decision threshold almost improves our business gains by factor of 2. .. _TunedThresholdClassifierCV_no_cv: Consideration regarding model refitting and cross-validation ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ In the above experiment, we used the default setting of the :class:`~sklearn.model_selection.TunedThresholdClassifierCV`. In particular, the cut-off point is tuned using a 5-fold stratified cross-validation. Also, the underlying predictive model is refitted on the entire training data once the cut-off point is chosen. These two strategies can be changed by providing the `refit` and `cv` parameters. For instance, one could provide a fitted `estimator` and set `cv="prefit"`, in which case the cut-off point is found on the entire dataset provided at fitting time. Also, the underlying classifier is not be refitted by setting `refit=False`. Here, we can try to do such experiment. .. GENERATED FROM PYTHON SOURCE LINES 407-412 .. code-block:: Python model.fit(X_train, y_train) tuned_model.set_params(cv="prefit", refit=False).fit(X_train, y_train) print(f"{tuned_model.best_threshold_=:0.2f}") .. rst-class:: sphx-glr-script-out .. code-block:: none tuned_model.best_threshold_=0.28 .. GENERATED FROM PYTHON SOURCE LINES 413-414 Then, we evaluate our model with the same approach as before: .. GENERATED FROM PYTHON SOURCE LINES 414-417 .. code-block:: Python title = "Tuned GBDT model without refitting and using the entire dataset" plot_roc_pr_curves(model, tuned_model, title=title) .. image-sg:: /auto_examples/model_selection/images/sphx_glr_plot_cost_sensitive_learning_003.png :alt: Tuned GBDT model without refitting and using the entire dataset, Precision-Recall curve, ROC curve, Objective score as a function of the decision threshold :srcset: /auto_examples/model_selection/images/sphx_glr_plot_cost_sensitive_learning_003.png :class: sphx-glr-single-img .. GENERATED FROM PYTHON SOURCE LINES 418-437 We observe the that the optimum cut-off point is different from the one found in the previous experiment. If we look at the right-hand side plot, we observe that the business gain has large plateau of near-optimal 0 gain for a large span of decision thresholds. This behavior is symptomatic of an overfitting. Because we disable cross-validation, we tuned the cut-off point on the same set as the model was trained on, and this is the reason for the observed overfitting. This option should therefore be used with caution. One needs to make sure that the data provided at fitting time to the :class:`~sklearn.model_selection.TunedThresholdClassifierCV` is not the same as the data used to train the underlying classifier. This could happen sometimes when the idea is just to tune the predictive model on a completely new validation set without a costly complete refit. When cross-validation is too costly, a potential alternative is to use a single train-test split by providing a floating number in range `[0, 1]` to the `cv` parameter. It splits the data into a training and testing set. Let's explore this option: .. GENERATED FROM PYTHON SOURCE LINES 437-439 .. code-block:: Python tuned_model.set_params(cv=0.75).fit(X_train, y_train) .. raw:: html

TunedThresholdClassifierCV(cv=0.75,
                               estimator=HistGradientBoostingClassifier(random_state=0),
                               refit=False,
                               scoring=make_scorer(credit_gain_score, response_method='predict', neg_label=good, pos_label=bad),
                               store_cv_results=True)

.. GENERATED FROM PYTHON SOURCE LINES 440-443 .. code-block:: Python title = "Tuned GBDT model without refitting and using the entire dataset" plot_roc_pr_curves(model, tuned_model, title=title) .. image-sg:: /auto_examples/model_selection/images/sphx_glr_plot_cost_sensitive_learning_004.png :alt: Tuned GBDT model without refitting and using the entire dataset, Precision-Recall curve, ROC curve, Objective score as a function of the decision threshold :srcset: /auto_examples/model_selection/images/sphx_glr_plot_cost_sensitive_learning_004.png :class: sphx-glr-single-img .. GENERATED FROM PYTHON SOURCE LINES 444-464 Regarding the cut-off point, we observe that the optimum is similar to the multiple repeated cross-validation case. However, be aware that a single split does not account for the variability of the fit/predict process and thus we are unable to know if there is any variance in the cut-off point. The repeated cross-validation averages out this effect. Another observation concerns the ROC and Precision-Recall curves of the tuned model. As expected, these curves differ from those of the vanilla model, given that we trained the underlying classifier on a subset of the data provided during fitting and reserved a validation set for tuning the cut-off point. Cost-sensitive learning when gains and costs are not constant ------------------------------------------------------------- As stated in [2]_, gains and costs are generally not constant in real-world problems. In this section, we use a similar example as in [2]_ for the problem of detecting fraud in credit card transaction records. The credit card dataset ^^^^^^^^^^^^^^^^^^^^^^^ .. GENERATED FROM PYTHON SOURCE LINES 464-467 .. code-block:: Python credit_card = fetch_openml(data_id=1597, as_frame=True, parser="pandas") credit_card.frame.info() .. rst-class:: sphx-glr-script-out .. code-block:: none RangeIndex: 284807 entries, 0 to 284806 Data columns (total 30 columns): # Column Non-Null Count Dtype --- ------ -------------- ----- 0 V1 284807 non-null float64 1 V2 284807 non-null float64 2 V3 284807 non-null float64 3 V4 284807 non-null float64 4 V5 284807 non-null float64 5 V6 284807 non-null float64 6 V7 284807 non-null float64 7 V8 284807 non-null float64 8 V9 284807 non-null float64 9 V10 284807 non-null float64 10 V11 284807 non-null float64 11 V12 284807 non-null float64 12 V13 284807 non-null float64 13 V14 284807 non-null float64 14 V15 284807 non-null float64 15 V16 284807 non-null float64 16 V17 284807 non-null float64 17 V18 284807 non-null float64 18 V19 284807 non-null float64 19 V20 284807 non-null float64 20 V21 284807 non-null float64 21 V22 284807 non-null float64 22 V23 284807 non-null float64 23 V24 284807 non-null float64 24 V25 284807 non-null float64 25 V26 284807 non-null float64 26 V27 284807 non-null float64 27 V28 284807 non-null float64 28 Amount 284807 non-null float64 29 Class 284807 non-null category dtypes: category(1), float64(29) memory usage: 63.3 MB .. GENERATED FROM PYTHON SOURCE LINES 468-471 The dataset contains information about credit card records from which some are fraudulent and others are legitimate. The goal is therefore to predict whether or not a credit card record is fraudulent. .. GENERATED FROM PYTHON SOURCE LINES 471-475 .. code-block:: Python columns_to_drop = ["Class"] data = credit_card.frame.drop(columns=columns_to_drop) target = credit_card.frame["Class"].astype(int) .. GENERATED FROM PYTHON SOURCE LINES 476-477 First, we check the class distribution of the datasets. .. GENERATED FROM PYTHON SOURCE LINES 477-479 .. code-block:: Python target.value_counts(normalize=True) .. rst-class:: sphx-glr-script-out .. code-block:: none Class 0 0.998273 1 0.001727 Name: proportion, dtype: float64 .. GENERATED FROM PYTHON SOURCE LINES 480-483 The dataset is highly imbalanced with fraudulent transaction representing only 0.17% of the data. Since we are interested in training a machine learning model, we should also make sure that we have enough samples in the minority class to train the model. .. GENERATED FROM PYTHON SOURCE LINES 483-485 .. code-block:: Python target.value_counts() .. rst-class:: sphx-glr-script-out .. code-block:: none Class 0 284315 1 492 Name: count, dtype: int64 .. GENERATED FROM PYTHON SOURCE LINES 486-490 We observe that we have around 500 samples that is on the low end of the number of samples required to train a machine learning model. In addition of the target distribution, we check the distribution of the amount of the fraudulent transactions. .. GENERATED FROM PYTHON SOURCE LINES 490-497 .. code-block:: Python fraud = target == 1 amount_fraud = data["Amount"][fraud] _, ax = plt.subplots() ax.hist(amount_fraud, bins=30) ax.set_title("Amount of fraud transaction") _ = ax.set_xlabel("Amount (€)") .. image-sg:: /auto_examples/model_selection/images/sphx_glr_plot_cost_sensitive_learning_005.png :alt: Amount of fraud transaction :srcset: /auto_examples/model_selection/images/sphx_glr_plot_cost_sensitive_learning_005.png :class: sphx-glr-single-img .. GENERATED FROM PYTHON SOURCE LINES 498-510 Addressing the problem with a business metric ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Now, we create the business metric that depends on the amount of each transaction. We define the cost matrix similarly to [2]_. Accepting a legitimate transaction provides a gain of 2% of the amount of the transaction. However, accepting a fraudulent transaction result in a loss of the amount of the transaction. As stated in [2]_, the gain and loss related to refusals (of fraudulent and legitimate transactions) are not trivial to define. Here, we define that a refusal of a legitimate transaction is estimated to a loss of 5€ while the refusal of a fraudulent transaction is estimated to a gain of 50€. Therefore, we define the following function to compute the total benefit of a given decision: .. GENERATED FROM PYTHON SOURCE LINES 510-524 .. code-block:: Python def business_metric(y_true, y_pred, amount): mask_true_positive = (y_true == 1) & (y_pred == 1) mask_true_negative = (y_true == 0) & (y_pred == 0) mask_false_positive = (y_true == 0) & (y_pred == 1) mask_false_negative = (y_true == 1) & (y_pred == 0) fraudulent_refuse = mask_true_positive.sum() * 50 fraudulent_accept = -amount[mask_false_negative].sum() legitimate_refuse = mask_false_positive.sum() * -5 legitimate_accept = (amount[mask_true_negative] * 0.02).sum() return fraudulent_refuse + fraudulent_accept + legitimate_refuse + legitimate_accept .. GENERATED FROM PYTHON SOURCE LINES 525-530 From this business metric, we create a scikit-learn scorer that given a fitted classifier and a test set compute the business metric. In this regard, we use the :func:`~sklearn.metrics.make_scorer` factory. The variable `amount` is an additional metadata to be passed to the scorer and we need to use :ref:`metadata routing ` to take into account this information. .. GENERATED FROM PYTHON SOURCE LINES 530-533 .. code-block:: Python sklearn.set_config(enable_metadata_routing=True) business_scorer = make_scorer(business_metric).set_score_request(amount=True) .. GENERATED FROM PYTHON SOURCE LINES 534-541 So at this stage, we observe that the amount of the transaction is used twice: once as a feature to train our predictive model and once as a metadata to compute the the business metric and thus the statistical performance of our model. When used as a feature, we are only required to have a column in `data` that contains the amount of each transaction. To use this information as metadata, we need to have an external variable that we can pass to the scorer or the model that internally routes this metadata to the scorer. So let's create this variable. .. GENERATED FROM PYTHON SOURCE LINES 541-543 .. code-block:: Python amount = credit_card.frame["Amount"].to_numpy() .. GENERATED FROM PYTHON SOURCE LINES 544-552 .. code-block:: Python from sklearn.model_selection import train_test_split data_train, data_test, target_train, target_test, amount_train, amount_test = ( train_test_split( data, target, amount, stratify=target, test_size=0.5, random_state=42 ) ) .. GENERATED FROM PYTHON SOURCE LINES 553-555 We first evaluate some baseline policies to serve as reference. Recall that class "0" is the legitimate class and class "1" is the fraudulent class. .. GENERATED FROM PYTHON SOURCE LINES 555-564 .. code-block:: Python from sklearn.dummy import DummyClassifier always_accept_policy = DummyClassifier(strategy="constant", constant=0) always_accept_policy.fit(data_train, target_train) benefit = business_scorer( always_accept_policy, data_test, target_test, amount=amount_test ) print(f"Benefit of the 'always accept' policy: {benefit:,.2f}€") .. rst-class:: sphx-glr-script-out .. code-block:: none Benefit of the 'always accept' policy: 221,445.07€ .. GENERATED FROM PYTHON SOURCE LINES 565-568 A policy that considers all transactions as legitimate would create a profit of around 220,000€. We make the same evaluation for a classifier that predicts all transactions as fraudulent. .. GENERATED FROM PYTHON SOURCE LINES 568-576 .. code-block:: Python always_reject_policy = DummyClassifier(strategy="constant", constant=1) always_reject_policy.fit(data_train, target_train) benefit = business_scorer( always_reject_policy, data_test, target_test, amount=amount_test ) print(f"Benefit of the 'always reject' policy: {benefit:,.2f}€") .. rst-class:: sphx-glr-script-out .. code-block:: none Benefit of the 'always reject' policy: -698,490.00€ .. GENERATED FROM PYTHON SOURCE LINES 577-591 Such a policy would entail a catastrophic loss: around 670,000€. This is expected since the vast majority of the transactions are legitimate and the policy would refuse them at a non-trivial cost. A predictive model that adapts the accept/reject decisions on a per transaction basis should ideally allow us to make a profit larger than the 220,000€ of the best of our constant baseline policies. We start with a logistic regression model with the default decision threshold at 0.5. Here we tune the hyperparameter `C` of the logistic regression with a proper scoring rule (the log loss) to ensure that the model's probabilistic predictions returned by its `predict_proba` method are as accurate as possible, irrespectively of the choice of the value of the decision threshold. .. GENERATED FROM PYTHON SOURCE LINES 591-603 .. code-block:: Python from sklearn.linear_model import LogisticRegression from sklearn.model_selection import GridSearchCV from sklearn.pipeline import make_pipeline from sklearn.preprocessing import StandardScaler logistic_regression = make_pipeline(StandardScaler(), LogisticRegression()) param_grid = {"logisticregression__C": np.logspace(-6, 6, 13)} model = GridSearchCV(logistic_regression, param_grid, scoring="neg_log_loss").fit( data_train, target_train ) model .. raw:: html

GridSearchCV(estimator=Pipeline(steps=[('standardscaler', StandardScaler()),
                                           ('logisticregression',
                                            LogisticRegression())]),
                 param_grid={'logisticregression__C': array([1.e-06, 1.e-05, 1.e-04, 1.e-03, 1.e-02, 1.e-01, 1.e+00, 1.e+01,
           1.e+02, 1.e+03, 1.e+04, 1.e+05, 1.e+06])},
                 scoring='neg_log_loss')

.. GENERATED FROM PYTHON SOURCE LINES 604-609 .. code-block:: Python print( "Benefit of logistic regression with default threshold: " f"{business_scorer(model, data_test, target_test, amount=amount_test):,.2f}€" ) .. rst-class:: sphx-glr-script-out .. code-block:: none Benefit of logistic regression with default threshold: 244,919.87€ .. GENERATED FROM PYTHON SOURCE LINES 610-623 The business metric shows that our predictive model with a default decision threshold is already winning over the baseline in terms of profit and it would be already beneficial to use it to accept or reject transactions instead of accepting all transactions. Tuning the decision threshold ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Now the question is: is our model optimum for the type of decision that we want to do? Up to now, we did not optimize the decision threshold. We use the :class:`~sklearn.model_selection.TunedThresholdClassifierCV` to optimize the decision given our business scorer. To avoid a nested cross-validation, we will use the best estimator found during the previous grid-search. .. GENERATED FROM PYTHON SOURCE LINES 623-630 .. code-block:: Python tuned_model = TunedThresholdClassifierCV( estimator=model.best_estimator_, scoring=business_scorer, thresholds=100, n_jobs=2, ) .. GENERATED FROM PYTHON SOURCE LINES 631-635 Since our business scorer requires the amount of each transaction, we need to pass this information in the `fit` method. The :class:`~sklearn.model_selection.TunedThresholdClassifierCV` is in charge of automatically dispatching this metadata to the underlying scorer. .. GENERATED FROM PYTHON SOURCE LINES 635-637 .. code-block:: Python tuned_model.fit(data_train, target_train, amount=amount_train) .. raw:: html

TunedThresholdClassifierCV(estimator=Pipeline(steps=[('standardscaler',
                                                          StandardScaler()),
                                                         ('logisticregression',
                                                          LogisticRegression(C=np.float64(100.0)))]),
                               n_jobs=2,
                               scoring=make_scorer(business_metric, response_method='predict'))

.. GENERATED FROM PYTHON SOURCE LINES 638-639 We observe that the tuned decision threshold is far away from the default 0.5: .. GENERATED FROM PYTHON SOURCE LINES 639-641 .. code-block:: Python print(f"Tuned decision threshold: {tuned_model.best_threshold_:.2f}") .. rst-class:: sphx-glr-script-out .. code-block:: none Tuned decision threshold: 0.03 .. GENERATED FROM PYTHON SOURCE LINES 642-647 .. code-block:: Python print( "Benefit of logistic regression with a tuned threshold: " f"{business_scorer(tuned_model, data_test, target_test, amount=amount_test):,.2f}€" ) .. rst-class:: sphx-glr-script-out .. code-block:: none Benefit of logistic regression with a tuned threshold: 249,433.39€ .. GENERATED FROM PYTHON SOURCE LINES 648-669 We observe that tuning the decision threshold increases the expected profit when deploying our model - as indicated by the business metric. It is therefore valuable, whenever possible, to optimize the decision threshold with respect to the business metric. Manually setting the decision threshold instead of tuning it ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ In the previous example, we used the :class:`~sklearn.model_selection.TunedThresholdClassifierCV` to find the optimal decision threshold. However, in some cases, we might have some prior knowledge about the problem at hand and we might be happy to set the decision threshold manually. The class :class:`~sklearn.model_selection.FixedThresholdClassifier` allows us to manually set the decision threshold. At prediction time, it behave as the previous tuned model but no search is performed during the fitting process. Note that here we use :class:`~sklearn.frozen.FrozenEstimator` to wrap the predictive model to avoid any refitting. Here, we will reuse the decision threshold found in the previous section to create a new model and check that it gives the same results. .. GENERATED FROM PYTHON SOURCE LINES 669-676 .. code-block:: Python from sklearn.frozen import FrozenEstimator from sklearn.model_selection import FixedThresholdClassifier model_fixed_threshold = FixedThresholdClassifier( estimator=FrozenEstimator(model), threshold=tuned_model.best_threshold_ ) .. GENERATED FROM PYTHON SOURCE LINES 677-682 .. code-block:: Python business_score = business_scorer( model_fixed_threshold, data_test, target_test, amount=amount_test ) print(f"Benefit of logistic regression with a tuned threshold: {business_score:,.2f}€") .. rst-class:: sphx-glr-script-out .. code-block:: none Benefit of logistic regression with a tuned threshold: 249,433.39€ .. GENERATED FROM PYTHON SOURCE LINES 683-692 We observe that we obtained the exact same results but the fitting process was much faster since we did not perform any hyper-parameter search. Finally, the estimate of the (average) business metric itself can be unreliable, in particular when the number of data points in the minority class is very small. Any business impact estimated by cross-validation of a business metric on historical data (offline evaluation) should ideally be confirmed by A/B testing on live data (online evaluation). Note however that A/B testing models is beyond the scope of the scikit-learn library itself. .. rst-class:: sphx-glr-timing **Total running time of the script:** (0 minutes 25.017 seconds) .. _sphx_glr_download_auto_examples_model_selection_plot_cost_sensitive_learning.py: .. only:: html .. container:: sphx-glr-footer sphx-glr-footer-example .. container:: binder-badge .. image:: images/binder_badge_logo.svg :target: https://mybinder.org/v2/gh/scikit-learn/scikit-learn/main?urlpath=lab/tree/notebooks/auto_examples/model_selection/plot_cost_sensitive_learning.ipynb :alt: Launch binder :width: 150 px .. container:: lite-badge .. image:: images/jupyterlite_badge_logo.svg :target: ../../lite/lab/index.html?path=auto_examples/model_selection/plot_cost_sensitive_learning.ipynb :alt: Launch JupyterLite :width: 150 px .. container:: sphx-glr-download sphx-glr-download-jupyter :download:`Download Jupyter notebook: plot_cost_sensitive_learning.ipynb ` .. container:: sphx-glr-download sphx-glr-download-python :download:`Download Python source code: plot_cost_sensitive_learning.py ` .. container:: sphx-glr-download sphx-glr-download-zip :download:`Download zipped: plot_cost_sensitive_learning.zip ` .. include:: plot_cost_sensitive_learning.recommendations .. only:: html .. rst-class:: sphx-glr-signature `Gallery generated by Sphinx-Gallery `_