분류 - 결정 트리

머신 러닝
공개

2025년 7월 27일

개요

  • 결정 노드가 많아지면 과적합이 발생할 수 있음
  • 가능한 한 적은 결정 노드로 높은 예측 정확도를 가지려면 데이터를 분류할 때 최대한 많은 데이터 세트가 해당 분류에 속할 수 있도록 규칙을 정해야 함
  • 균일하게 데이터 세트를 구성할 수 있도록 분할하는 것이 필요
  • 균일도만 신경쓰면 되기 때문에 전처리 작업이 필요 없음

파라미터

  • min_samples_split: 노드를 분할하기 위한 최소한의 샘플 수.
  • min_samples_leaf: 말단 노드가 되기 위한 최소한의 샘플 수. 비대칭적 데이터의 경우 특정 클래스의 데이터가 극도로 작을 수 있으므로 작게 설정 필요
  • max_features: 분할을 고려할 feature의 수. default는 None으로 모든 feature를 고려함
  • max_depth
  • max_leaf_nodes

시각화

from sklearn.tree import DecisionTreeClassifier, export_graphviz
from sklearn.datasets import load_iris
from sklearn.model_selection import train_test_split
import graphviz
import warnings

warnings.filterwarnings('ignore')
dt_clf = DecisionTreeClassifier()

iris_data = load_iris()
X_train, X_test, y_train, y_test = train_test_split(iris_data.data, iris_data.target, test_size=0.2)

dt_clf.fit(X_train, y_train)
DecisionTreeClassifier()
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export_graphviz(dt_clf, out_file="tree.dot", class_names=iris_data.target_names, feature_names=iris_data.feature_names, impurity=True, filled=True)
with open("tree.dot") as f:
    dot_graph = f.read()
graphviz.Source(dot_graph)

import seaborn as sns
import numpy as np

for name, value in zip(iris_data.feature_names, dt_clf.feature_importances_):
    print(f'{name}: {value:.3f}')
sns.barplot(x=dt_clf.feature_importances_, y=iris_data.feature_names)
sepal length (cm): 0.000
sepal width (cm): 0.017
petal length (cm): 0.544
petal width (cm): 0.439

examples

import pandas as pd


def get_new_feature_name_df(old):
    df = pd.DataFrame(data=old.groupby('column_name').cumcount(), columns=['dup_cnt'])
    df = df.reset_index()
    new_df = pd.merge(old.reset_index(), df, how='outer')
    new_df['column_name'] = new_df[['column_name', 'dup_cnt']].apply(lambda x: x[0] + '_' + str(x[1]) if x[1] > 0 else x[0], axis=1)
    new_df = new_df.drop(['index'], axis=1)
    return new_df

def get_human_dataset():
    feature_name_df = pd.read_csv('_data/human_activity/features.txt', sep='\s+', header=None, names=['column_index', 'column_name'])
    new_feature_name_df = get_new_feature_name_df(feature_name_df)
    feature_name = new_feature_name_df.iloc[:, 1].values.tolist()

    X_train = pd.read_csv('_data/human_activity/train/X_train.txt', sep='\s+', names=feature_name)
    X_test = pd.read_csv('_data/human_activity/test/X_test.txt', sep='\s+', names=feature_name)

    y_train = pd.read_csv('_data/human_activity/train/y_train.txt', sep='\s+', header=None, names=['action'])
    y_test = pd.read_csv('_data/human_activity/test/y_test.txt', sep='\s+', header=None, names=['action'])

    return X_train, X_test, y_train, y_test

X_train, X_test, y_train, y_test = get_human_dataset()
X_train.info()
<class 'pandas.core.frame.DataFrame'>
RangeIndex: 7352 entries, 0 to 7351
Columns: 561 entries, tBodyAcc-mean()-X to angle(Z,gravityMean)
dtypes: float64(561)
memory usage: 31.5 MB
y_train['action'].value_counts()
action
6    1407
5    1374
4    1286
1    1226
2    1073
3     986
Name: count, dtype: int64

default 파라미터 예측

from sklearn.metrics import accuracy_score

dt_clf = DecisionTreeClassifier()
dt_clf.fit(X_train, y_train)
pred = dt_clf.predict(X_test)
accuracy = accuracy_score(y_test, pred)
accuracy
0.8608754665761792

하이퍼파라미터 최적화

from sklearn.model_selection import GridSearchCV

params = {
    'max_depth': [6, 8, 10, 12, 16, 20, 24],
    'min_samples_split': [16]
}

grid_cv = GridSearchCV(dt_clf, param_grid=params, scoring='accuracy',cv=5, verbose=1)
grid_cv.fit(X_train, y_train)

cv_results_df = pd.DataFrame(grid_cv.cv_results_)
cv_results_df[['param_max_depth', 'mean_test_score']]
cv_results_df
Fitting 5 folds for each of 7 candidates, totalling 35 fits
mean_fit_time std_fit_time mean_score_time std_score_time param_max_depth param_min_samples_split params split0_test_score split1_test_score split2_test_score split3_test_score split4_test_score mean_test_score std_test_score rank_test_score
0 1.730651 0.071066 0.004747 0.000612 6 16 {'max_depth': 6, 'min_samples_split': 16} 0.812373 0.870156 0.834694 0.865306 0.869388 0.850383 0.023090 2
1 2.199078 0.058100 0.004974 0.000507 8 16 {'max_depth': 8, 'min_samples_split': 16} 0.812373 0.831407 0.842177 0.870068 0.890476 0.849300 0.027790 3
2 2.591271 0.101800 0.004741 0.000625 10 16 {'max_depth': 10, 'min_samples_split': 16} 0.813732 0.819850 0.837415 0.888435 0.896599 0.851206 0.034711 1
3 2.873773 0.100605 0.005099 0.001333 12 16 {'max_depth': 12, 'min_samples_split': 16} 0.783821 0.817811 0.853741 0.885714 0.882313 0.844680 0.039008 5
4 3.446341 0.244669 0.004722 0.000739 16 16 {'max_depth': 16, 'min_samples_split': 16} 0.803535 0.810333 0.842177 0.878231 0.887075 0.844270 0.034062 6
5 3.307252 0.254648 0.004365 0.000410 20 16 {'max_depth': 20, 'min_samples_split': 16} 0.794018 0.811693 0.831293 0.887075 0.891837 0.843183 0.039608 7
6 3.243996 0.227535 0.004434 0.000333 24 16 {'max_depth': 24, 'min_samples_split': 16} 0.796737 0.809653 0.846259 0.882313 0.895238 0.846040 0.038707 4 
best_df_clf = grid_cv.best_estimator_

ftr_importances_values = best_df_clf.feature_importances_
ftr_importances = pd.Series(ftr_importances_values, index=X_train.columns)
ftr_top20 = ftr_importances.sort_values(ascending=False)[:20]
sns.barplot(x=ftr_top20, y=ftr_top20.index)

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