EDA(Exploratory Data Analysis)探索性数据分析
EDA(Exploratory Data Analysis)中文名称为探索性数据分析，是为了在特征工程或模型开发之前对数据有个基本的了解。数据类型通常分为两类：连续类型和离散类型，特征类型不同，我们探索的内容也不同。
1. 特征类型
1.1 连续型特征
定义：取值为数值类型且数值之间的大小具有实际含义。例如：收入。对于连续型变量，需要进行EDA的内容包括：
- 缺失值
- 均值
- 方差
- 标准差
- 最大值
- 最小值
- 中位数
- 众数
- 四分位数
- 偏度
- 最大取值类别对应的样本数
1.2 离散型特征
定义：不具有数学意义的特征。如：性别。对于离散型变量，需要进行EDA的内容包括：
- 缺失值
- 众数
- 取值个数
- 最大取值类别对应的样本数
- 每个取值对应的样本数
2. EDA目的
通过EDA，需要达到以下几个目的：
（1）可以有效发现变量类型、分布趋势、缺失值、异常值等。
（2）缺失值处理：（i）删除缺失值较多的列，通常缺失超过50%的列需要删除；（ii）缺失值填充。对于离散特征，通常将NAN单独作为一个类别；对于连续特征，通常使用均值、中值、0或机器学习算法进行填充。具体填充方法因业务的不同而不同。
（3）异常值处理（主要针对连续特征）。如：Winsorizer方法处理。
（4）类别合并（主要针对离散特征）。如果某个取值对应的样本个数太少，就需要将该取值与其他值合并。因为样本过少会使数据的稳定性变差，且不具有统计意义，可能导致结论错误。由于展示空间有限，通常选择取值个数最少或最多的多个取值进行展示。
（5）删除取值单一的列。
（6）删除最大类别取值数量占比超过阈值的列。
3.实验
3.1 统计变量类型、分布趋势、缺失值、异常值等
#!/usr/bin/python
import pandas as pd
import numpy as np
def getTopValues(series, top = 5, reverse = False):
"""Get top/bottom n values
Args:
series (Series): data series
top (number): number of top/bottom n values
reverse (bool): it will return bottom n values if True is given
Returns:
Series: Series of top/bottom n values and percentage. ['value:percent', None]
"""
itype = 'top'
counts = series.value_counts()
counts = list(zip(counts.index, counts, counts.divide(series.size)))
if reverse:
counts.reverse()
itype = 'bottom'
template = "{0[0]}:{0[2]:.2%}"
indexs = [itype + str(i + 1) for i in range(top)]
values = [template.format(counts[i]) if i < len(counts) else None for i in range(top)]
return pd.Series(values, index = indexs)
def getDescribe(series, percentiles = [.25, .5, .75]):
"""Get describe of series
Args:
series (Series): data series
percentiles: the percentiles to include in the output
Returns:
Series: the describe of data include mean, std, min, max and percentiles
"""
d = series.describe(percentiles)
return d.drop('count')
def countBlank(series, blanks = []):
"""Count number and percentage of blank values in series
Args:
series (Series): data series
blanks (list): list of blank values
Returns:
number: number of blanks
str: the percentage of blank values
"""
if len(blanks)>0:
isnull = series.replace(blanks, None).isnull()
else:
isnull = series.isnull()
n = isnull.sum()
ratio = isnull.mean()
return (n, "{0:.2%}".format(ratio))
def isNumeric(series):
"""Check if the series's type is numeric
Args:
series (Series): data series
Returns:
bool
"""
return series.dtype.kind in 'ifc'
def detect(dataframe):
""" Detect data
Args:
dataframe (DataFrame): data that will be detected
Returns:
DataFrame: report of detecting
"""
numeric_rows = []
category_rows = []
for name, series in dataframe.items():
# 缺失值比例
nblank, pblank = countBlank(series)
# 最大类别取值占比
biggest_category_percentage = series.value_counts(normalize=True, dropna=False).values[0] * 100
if isNumeric(series):
desc = getDescribe(
series,
percentiles=[.01, .1, .5, .75, .9, .99]
)
details = desc.tolist()
details_index = ['mean', 'std', 'min', '1%', '10%', '50%', '75%', '90%', '99%', 'max']
row = pd.Series(
index=['type', 'size', 'missing', 'unique', 'biggest_category_percentage', 'skew'] + details_index,
data=[series.dtype, series.size, pblank, series.nunique(), biggest_category_percentage, series.skew()] + details
)
row.name = name
numeric_rows.append(row)
else:
top5 = getTopValues(series)
bottom5 = getTopValues(series, reverse=True)
details = top5.tolist() + bottom5[::-1].tolist()
details_index = ['top1', 'top2', 'top3', 'top4', 'top5', 'bottom5', 'bottom4', 'bottom3', 'bottom2', 'bottom1']
row = pd.Series(
index=['type', 'size', 'missing', 'unique', 'biggest_category_percentage'] + details_index,
data=[series.dtype, series.size, pblank, series.nunique(), biggest_category_percentage] + details
)
row.name = name
category_rows.append(row)
return pd.DataFrame(numeric_rows), pd.DataFrame(category_rows)
demo(数据来自：https://www.kaggle.com/competitions/home-credit-default-risk/data）
import os
import eda
import pandas as pd
import numpy as np
data_dir = "./"
df = pd.read_csv(os.path.join(data_dir, "bureau.csv"))
numeric_df, category_df = eda.detect(df)
3.2 缺失值处理（示例）
#连续特征
df[col].fillna(-df[col].mean(), inplace=True)
#离散特征
df[col].fillna('nan', inplace=True)
3.3 删除无用特征
def get_del_columns(df):
del_columns = {}
for index, row in df.iterrows():
if row["unique"] < 2:
del_columns[row["Feature"]] = "取值单一"
continue
if row["missing"] > 90:
del_columns[row["Feature"]] = "缺失值数量大于90%"
continue
if row["biggest_category_percentage"] > 99:
del_columns[row["Feature"]] = "取值最多的类别占比超过99%"
continue
del_columns[row["Feature"]] = "正常"
return del_columns
3.4 异常值处理
Winsorizer算法（定义某个变量的上界和下界，取值超过边界的话会用边界的值取代）：
class Winsorizer():
"""Performs Winsorization 1->1*
Warning: this class should not be used directly.
"""
def __init__(self,trim_quantile=0.0):
self.trim_quantile=trim_quantile
self.winsor_lims=None
def train(self,X):
# get winsor limits
self.winsor_lims=np.ones([2,X.shape[1]])*np.inf
self.winsor_lims[0,:]=-np.inf
if self.trim_quantile>0:
for i_col in np.arange(X.shape[1]):
lower=np.percentile(X[:,i_col],self.trim_quantile*100)
upper=np.percentile(X[:,i_col],100-self.trim_quantile*100)
self.winsor_lims[:,i_col]=[lower,upper]
def trim(self,X):
X_=X.copy()
X_=np.where(X>self.winsor_lims[1,:],np.tile(self.winsor_lims[1,:],[X.shape[0],1]),np.where(X<self.winsor_lims[0,:],np.tile(self.winsor_lims[0,:],[X.shape[0],1]),X))
return X_
winsorizer = Winsorizer (0.1)
a=np.random.random((10,2))
print("转化前: ", a)
winsorizer.train(a)
print("上界和下界: ", winsorizer.winsor_lims)
b = winsorizer.trim(a)
print("转化后: ", b)
4.总结
这篇文章只总结了EDA的常用做法，实际应用过程中还需要根据具体业务来做调整。