Techniques for Feature Selection and Parameter Optimization¶
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%pylab inline
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import pandas as pd
import numpy as np
import matplotlib.pyplot as plt
Import titanic data using pandas¶
Modified version of the "Titanic" data can be found at: http://facweb.cs.depaul.edu/mobasher/classes/csc478/Data/titanic-trimmed.csv. Original unmodified Titanic data is available at CMU StatLib.
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url = "http://facweb.cs.depaul.edu/mobasher/classes/csc478/Data/titanic-trimmed.csv"
titanic = pd.read_csv(url)
titanic.head(10)
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titanic.describe(include="all")
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Handling missing variables¶
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titanic[titanic.age.isnull()].shape
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age_mean = titanic.age.mean()
titanic.age.fillna(age_mean, axis=0, inplace=True)
titanic.dropna(axis=0, inplace=True)
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titanic.shape
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titanic.set_index('pid', drop=True, inplace=True)
titanic.head()
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Creating dummy variables for categorical features¶
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titanic_ssf = pd.get_dummies(titanic)
titanic_ssf.head(10)
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titanic_names = titanic_ssf.columns.values
titanic_names
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y = titanic_ssf['survived']
X = titanic_ssf[titanic_names[1:]]
X.head()
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titanic_ssf.describe()
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titanic_ssf.describe().T
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Build the training and testing dataset¶
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from sklearn.cross_validation import train_test_split
X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.2, random_state=33)
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# Now let's train the decision tree on the training data
from sklearn import tree
dt = tree.DecisionTreeClassifier(criterion='entropy')
dt = dt.fit(X_train, y_train)
A versatile function to measure performance of a calssification model¶
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from sklearn import metrics
def measure_performance(X, y, clf, show_accuracy=True, show_classification_report=True, show_confussion_matrix=True):
y_pred = clf.predict(X)
if show_accuracy:
print "Accuracy:{0:.3f}".format(metrics.accuracy_score(y, y_pred)),"\n"
if show_classification_report:
print "Classification report"
print metrics.classification_report(y, y_pred),"\n"
if show_confussion_matrix:
print "Confussion matrix"
print metrics.confusion_matrix(y, y_pred),"\n"
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from sklearn import metrics
measure_performance(X_test, y_test, dt, show_confussion_matrix=False, show_classification_report=True)
Feature Selection¶
Select the top 30% of the most important features, using a chi2 test¶
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from sklearn import feature_selection
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fs = feature_selection.SelectPercentile(feature_selection.chi2, percentile=30)
X_train_fs = fs.fit_transform(X_train, y_train)
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np.set_printoptions(suppress=True, precision=2, linewidth=80)
print fs.get_support()
print fs.scores_
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X.columns.values
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print X.columns[fs.get_support()].values
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for i in range(len(X.columns.values)):
if fs.get_support()[i]:
print X.columns.values[i],'\t', fs.scores_[i]
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print X_train_fs
Evaluate performance with the new feature set on test data¶
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dt = tree.DecisionTreeClassifier(criterion='entropy')
dt.fit(X_train_fs, y_train)
X_test_fs = fs.transform(X_test)
measure_performance(X_test_fs, y_test, dt, show_confussion_matrix=False, show_classification_report=False)
To do feature selection more systematically, we need to find the best percentile using cross-validation¶
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from sklearn import cross_validation
dt = tree.DecisionTreeClassifier(criterion='entropy')
percentiles = range(1, 100, 5)
results = []
for i in range(1, 100, 5):
fs = feature_selection.SelectPercentile(feature_selection.chi2, percentile=i)
X_train_fs = fs.fit_transform(X_train, y_train)
scores = cross_validation.cross_val_score(dt, X_train_fs, y_train, cv=5)
print i,scores.mean()
results = np.append(results, scores.mean())
optimal_percentile = np.where(results == results.max())[0]
print "Optimal percentile of features:{0}".format(percentiles[optimal_percentile]), "\n"
optimal_num_features = int(percentiles[optimal_percentile]*len(X.columns)/100)
print "Optimal number of features:{0}".format(optimal_num_features), "\n"
# Plot percentile of features VS. cross-validation scores
import pylab as pl
pl.figure()
pl.xlabel("Percentage of features selected")
pl.ylabel("Cross validation accuracy")
pl.plot(percentiles,results)
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Evaluate our best number of features on the test set¶
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fs = feature_selection.SelectKBest(feature_selection.chi2, optimal_num_features)
X_train_fs = fs.fit_transform(X_train, y_train)
dt = tree.DecisionTreeClassifier(criterion='entropy')
dt.fit(X_train_fs, y_train)
X_test_fs = fs.transform(X_test)
measure_performance(X_test_fs, y_test, dt, show_confussion_matrix=False, show_classification_report=True)
Model selection¶
Exploring and comparing model parameters¶
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print dt.get_params()
Let's first focus on "criterion' parameter and find the best one¶
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dt = tree.DecisionTreeClassifier(criterion='entropy')
scores = cross_validation.cross_val_score(dt, X_train_fs, y_train, cv=5)
print "Entropy criterion accuracy on cv: {0:.3f}".format(scores.mean())
dt = tree.DecisionTreeClassifier(criterion='gini')
scores = cross_validation.cross_val_score(dt, X_train_fs, y_train, cv=5)
print "Gini criterion accuracy on cv: {0:.3f}".format(scores.mean())
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dt.fit(X_train_fs, y_train)
X_test_fs = fs.transform(X_test)
measure_performance(X_test_fs, y_test, dt, show_confussion_matrix=False, show_classification_report=True)
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dt = tree.DecisionTreeClassifier(criterion='entropy')
dt.fit(X_train, y_train)
measure_performance(X_test, y_test, dt, show_confussion_matrix=False, show_classification_report=False)
Another parameter of decision tree that can have an impact on accuracy is 'max-depth'¶
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dt = tree.DecisionTreeClassifier(criterion='entropy')
dt.set_params(max_depth=5)
dt.fit(X_train, y_train)
measure_performance(X_test, y_test, dt, show_confussion_matrix=False, show_classification_report=False)
But, again, we need a more systematic way to explore the space of values for each parameter. The following is a general function that performs cross-validation using a range of values for a specified parameter of a model¶
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from sklearn.cross_validation import KFold
def calc_params(X, y, clf, param_values, param_name, K):
# Convert input to Numpy arrays
X = np.array(X)
y = np.array(y)
# initialize training and testing scores with zeros
train_scores = np.zeros(len(param_values))
test_scores = np.zeros(len(param_values))
# iterate over the different parameter values
for i, param_value in enumerate(param_values):
print param_name, ' = ', param_value
# set classifier parameters
clf.set_params(**{param_name:param_value})
# initialize the K scores obtained for each fold
k_train_scores = np.zeros(K)
k_test_scores = np.zeros(K)
# create KFold cross validation
cv = KFold(len(X), K, shuffle=True, random_state=0)
# iterate over the K folds
for j, (train, test) in enumerate(cv):
# fit the classifier in the corresponding fold
# and obtain the corresponding accuracy scores on train and test sets
clf.fit([X[k] for k in train], y[train])
k_train_scores[j] = clf.score([X[k] for k in train], y[train])
k_test_scores[j] = clf.score([X[k] for k in test], y[test])
# store the mean of the K fold scores
train_scores[i] = np.mean(k_train_scores)
test_scores[i] = np.mean(k_test_scores)
# plot the training and testing scores in a log scale
plt.plot(param_values, train_scores, label='Train', alpha=0.4, lw=2, c='b')
plt.plot(param_values, test_scores, label='X-Val', alpha=0.4, lw=2, c='g')
plt.legend(loc=7)
plt.xlabel(param_name + " values")
plt.ylabel("Mean cross validation accuracy")
# return the training and testing scores on each parameter value
return train_scores, test_scores
Now we can explore the impact of max-depth more systematically¶
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# Let's create an evenly spaced range of numbers in a specified interval
md = np.linspace(1, 40, 20)
md = np.array([int(e) for e in md])
print md
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train_scores, test_scores = calc_params(X_train, y_train, dt, md, 'max_depth', 5)
max_depth = 3 seems to work best; larger values seem to lead to over-fitting.¶
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dt = tree.DecisionTreeClassifier(criterion='entropy')
dt.set_params(max_depth=3)
dt.fit(X_train, y_train)
measure_performance(X_test, y_test, dt, show_confussion_matrix=False, show_classification_report=False)
Another parameter of decision tree that's important is the min number of samples allowed at a leaf node¶
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msl = np.linspace(1, 30, 15)
msl = np.array([int(e) for e in msl])
dt = tree.DecisionTreeClassifier(criterion='entropy')
train_scores, test_scores = calc_params(X_train, y_train, dt, msl, 'min_samples_leaf', 5)
Looks like min_samples_leaf around 11 seems like a good choice¶
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dt = tree.DecisionTreeClassifier(criterion='entropy')
dt.set_params(min_samples_leaf=11)
dt.fit(X_train, y_train)
measure_performance(X_test, y_test, dt, show_confussion_matrix=False, show_classification_report=False)
Grid Search allows us to more systemiatically explore different combinations of multiple parameters¶
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from sklearn.grid_search import GridSearchCV
dt = tree.DecisionTreeClassifier()
parameters = {
'criterion': ['entropy','gini'],
'max_depth': np.linspace(1, 20, 10),
'min_samples_leaf': np.linspace(1, 30, 15),
'min_samples_split': np.linspace(2, 20, 10)
}
gs = GridSearchCV(dt, parameters, verbose=1, cv=5)
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%time _ = gs.fit(X_train, y_train)
gs.best_params_, gs.best_score_
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dt = tree.DecisionTreeClassifier(criterion='gini', max_depth=3, min_samples_leaf=3, min_samples_split=2)
dt.fit(X_train, y_train)
measure_performance(X_test, y_test, dt, show_confussion_matrix=False, show_classification_report=True)
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