Cross Validation Scores¶
Generally we determine whether a given model is optimal by looking at it’s F1, precision, recall, and accuracy (for classification), or it’s coefficient of determination (R2) and error (for regression). However, real world data is often distributed somewhat unevenly, meaning that the fitted model is likely to perform better on some sections of the data than on others. Yellowbrick’s CVScores
visualizer enables us to visually explore these variations in performance using different cross validation strategies.
Cross Validation¶
Crossvalidation starts by shuffling the data (to prevent any unintentional ordering errors) and splitting it into k folds. Then k models are fit on \(\frac{k1} {k}\) of the data (called the training split) and evaluated on \(\frac {1} {k}\) of the data (called the test split). The results from each evaluation are averaged together for a final score, then the final model is fit on the entire dataset for operationalization.
In Yellowbrick, the CVScores
visualizer displays crossvalidated scores as a bar chart (one bar for each fold) with the average score across all folds plotted as a horizontal dotted line.
Classification¶
In the following example, we show how to visualize crossvalidated scores for a classification model. After loading our occupancy data as a DataFrame
, we created a StratifiedKFold
crossvalidation strategy to ensure all of our classes in each split are represented with the same proportion. We then fit the CVScores
visualizer using the f1_weighted
scoring metric as opposed to the default metric, accuracy, to get a better sense of the relationship of precision and recall in our classifier across all of our folds.
from sklearn.model_selection import StratifiedKFold
from sklearn.naive_bayes import MultinomialNB
from yellowbrick.datasets import load_occupancy
from yellowbrick.model_selection import CVScores
# Load the classification dataset
X, y = load_occupancy()
# Create a crossvalidation strategy
cv = StratifiedKFold(n_splits=12, random_state=42)
# Instantiate the classification model and visualizer
model = MultinomialNB()
visualizer = CVScores(model, cv=cv, scoring='f1_weighted')
visualizer.fit(X, y) # Fit the data to the visualizer
visualizer.poof() # Draw/show/poof the data
(Source code, png, pdf)
Our resulting visualization shows that while our average crossvalidation score is quite high, there are some splits for which our fitted MultinomialNB
classifier performs significantly less well.
Regression¶
In this next example we show how to visualize crossvalidated scores for a regression model. After loading our energy data as a DataFrame
, we instantiated a simple KFold
crossvalidation strategy. We then fit the CVScores
visualizer using the r2
scoring metric, to get a sense of the coefficient of determination for our regressor across all of our folds.
from sklearn.linear_model import Ridge
from sklearn.model_selection import KFold
from yellowbrick.datasets import load_energy
from yellowbrick.model_selection import CVScores
# Load the regression dataset
X, y = load_energy()
# Instantiate the regression model and visualizer
cv = KFold(n_splits=12, random_state=42)
model = Ridge()
visualizer = CVScores(model, cv=cv, scoring='r2')
visualizer.fit(X, y) # Fit the data to the visualizer
visualizer.poof() # Draw/show/poof the data
(Source code, png, pdf)
As with our classification CVScores
visualization, our regression visualization suggests that our Ridge
regressor performs very well (e.g. produces a high coefficient of determination) across nearly every fold, resulting in another fairly high overall R2 score.
API Reference¶
Implements crossvalidation score plotting for model selection.

class
yellowbrick.model_selection.cross_validation.
CVScores
(model, ax=None, cv=None, scoring=None, color=None, **kwargs)[source]¶ Bases:
yellowbrick.base.ModelVisualizer
CVScores displays crossvalidated scores as a bar chart, with the average of the scores plotted as a horizontal line.
 Parameters
 modela scikitlearn estimator
An object that implements
fit
andpredict
, can be a classifier, regressor, or clusterer so long as there is also a valid associated scoring metric. Note that the object is cloned for each validation. axmatplotlib.Axes object, optional
The axes object to plot the figure on.
 cvint, crossvalidation generator or an iterable, optional
Determines the crossvalidation splitting strategy. Possible inputs for cv are:
None, to use the default 3fold crossvalidation,
integer, to specify the number of folds.
An object to be used as a crossvalidation generator.
An iterable yielding train/test splits.
See the scikitlearn crossvalidation guide for more information on the possible strategies that can be used here.
 scoringstring, callable or None, optional, default: None
A string or scorer callable object / function with signature
scorer(estimator, X, y)
.See scikitlearn crossvalidation guide for more information on the possible metrics that can be used.
 color: string
Specify color for barchart
 kwargsdict
Keyword arguments that are passed to the base class and may influence the visualization as defined in other Visualizers.
 Attributes
 ——
 cv_scores_ndarray shape (n_splits, )
The crossvalidated scores from each subsection of the data
 cv_scores_mean_float
Average crossvalidated score across all subsections of the data
Notes
This visualizer is a wrapper for sklearn.model_selection.cross_val_score.
Refer to the scikitlearn crossvalidation guide for more details.
Examples
>>> from sklearn import datasets, svm >>> iris = datasets.load_iris() >>> clf = svm.SVC(kernel='linear', C=1) >>> X = iris.data >>> y = iris.target >>> visualizer = CVScores(model=clf, cv=5, scoring='f1_macro') >>> visualizer.fit(X,y) >>> visualizer.poof()

draw
(self, **kwargs)[source]¶ Creates the bar chart of the crossvalidated scores generated from the fit method and places a dashed horizontal line that represents the average value of the scores.

finalize
(self, **kwargs)[source]¶ Add the title, legend, and other visual final touches to the plot.

fit
(self, X, y, **kwargs)[source]¶ Fits the learning curve with the wrapped model to the specified data. Draws training and test score curves and saves the scores to the estimator.
 Parameters
 Xarraylike, shape (n_samples, n_features)
Training vector, where n_samples is the number of samples and n_features is the number of features.
 yarraylike, shape (n_samples) or (n_samples, n_features), optional
Target relative to X for classification or regression; None for unsupervised learning.
 Returns
 selfinstance