API Reference

class feature_engine.encoding.WoEEncoder(variables=None, ignore_format=False)[source]

The WoERatioCategoricalEncoder() replaces categories by the weight of evidence (WoE). The WoE was used primarily in the financial sector to create credit risk scorecards.

The encoder will encode only categorical variables by default (type ‘object’ or ‘categorical’). You can pass a list of variables to encode. Alternatively, the encoder will find and encode all categorical variables (type ‘object’ or ‘categorical’).

With ignore_format=True you have the option to encode numerical variables as well. The procedure is identical, you can either enter the list of variables to encode, or the transformer will automatically select all variables.

The encoder first maps the categories to the weight of evidence for each variable (fit). The encoder then transforms the categories into the mapped numbers (transform).


This categorical encoding is exclusive for binary classification.

The weight of evidence is given by:

\[log( p(X=xj|Y = 1) / p(X=xj|Y=0) )\]

The WoE is determined as follows:

We calculate the percentage positive cases in each category of the total of all positive cases. For example 20 positive cases in category A out of 100 total positive cases equals 20 %. Next, we calculate the percentage of negative cases in each category respect to the total negative cases, for example 5 negative cases in category A out of a total of 50 negative cases equals 10%. Then we calculate the WoE by dividing the category percentages of positive cases by the category percentage of negative cases, and take the logarithm, so for category A in our example WoE = log(20/10).


  • If WoE values are negative, negative cases supersede the positive cases.

  • If WoE values are positive, positive cases supersede the negative cases.

  • And if WoE is 0, then there are equal number of positive and negative examples.

Encoding into WoE:

  • Creates a monotonic relationship between the encoded variable and the target

  • Returns variables in a similar scale


The log(0) is not defined and the division by 0 is not defined. Thus, if any of the terms in the WoE equation are 0 for a given category, the encoder will return an error. If this happens, try grouping less frequent categories.

variables: list, default=None

The list of categorical variables that will be encoded. If None, the encoder will find and transform all variables of type object or categorical by default. You can also make the transformer accept numerical variables, see the next parameter.

ignore_format: bool, default=False

Whether the format in which the categorical variables are cast should be ignored. If false, the encoder will automatically select variables of type object or categorical, or check that the variables entered by the user are of type object or categorical. If True, the encoder will select all variables or accept all variables entered by the user, including those cast as numeric.



Dictionary with the WoE per variable.


The group of variables that will be transformed.


The number of features in the train set used in fit.

See also



For details on the calculation of the weight of evidence visit: https://www.listendata.com/2015/03/weight-of-evidence-woe-and-information.html

In credit scoring, continuous variables are also transformed using the WoE. To do this, first variables are sorted into a discrete number of bins, and then these bins are encoded with the WoE as explained here for categorical variables. You can do this by combining the use of the equal width, equal frequency or arbitrary discretisers.

NAN are introduced when encoding categories that were not present in the training dataset. If this happens, try grouping infrequent categories using the RareLabelEncoder().



Learn the WoE per category, per variable.


Encode the categories to numbers.


Fit to the data, then transform it.


Encode the numbers into the original categories.

fit(X, y)[source]

Learn the WoE.

X: pandas dataframe of shape = [n_samples, n_features]

The training input samples. Can be the entire dataframe, not just the categorical variables.

y: pandas series.

Target, must be binary.

  • If the input is not the Pandas DataFrame.

  • If user enters non-categorical variables (unless ignore_format is True)

  • If there are no categorical variables in df or df is empty

  • If variable(s) contain null values.

  • If y is not binary with values 0 and 1.

  • If p(0) = 0 or p(1) = 0.


Convert the encoded variable back to the original values.

X: pandas dataframe of shape = [n_samples, n_features].

The transformed dataframe.

X: pandas dataframe of shape = [n_samples, n_features].

The un-transformed dataframe, with the categorical variables containing the original values.


DataFrame ..


If the input is not a Pandas DataFrame

  • If the variable(s) contain null values

  • If the df has different number of features than the df used in fit()


Replace categories with the learned parameters.

X: pandas dataframe of shape = [n_samples, n_features].

The dataset to transform.

X: pandas dataframe of shape = [n_samples, n_features].

The dataframe containing the categories replaced by numbers.


DataFrame ..


If the input is not a Pandas DataFrame

  • If the variable(s) contain null values

  • If the df has different number of features than the df used in fit()


If after encoding, NAN were introduced.


import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
from sklearn.model_selection import train_test_split

from feature_engine.encoding import WoEEncoder, RareLabelEncoder

# Load dataset
def load_titanic():
        data = pd.read_csv('https://www.openml.org/data/get_csv/16826755/phpMYEkMl')
        data = data.replace('?', np.nan)
        data['cabin'] = data['cabin'].astype(str).str[0]
        data['pclass'] = data['pclass'].astype('O')
        data['embarked'].fillna('C', inplace=True)
        return data

data = load_titanic()

# Separate into train and test sets
X_train, X_test, y_train, y_test = train_test_split(
                data.drop(['survived', 'name', 'ticket'], axis=1),
                data['survived'], test_size=0.3, random_state=0)

# set up a rare label encoder
rare_encoder = RareLabelEncoder(tol=0.03, n_categories=2, variables=['cabin', 'pclass', 'embarked'])

# fit and transform data
train_t = rare_encoder.fit_transform(X_train)
test_t = rare_encoder.transform(X_train)

# set up a weight of evidence encoder
woe_encoder = WoEEncoder(variables=['cabin', 'pclass', 'embarked'])

# fit the encoder
woe_encoder.fit(train_t, y_train)

# transform
train_t = woe_encoder.transform(train_t)
test_t = woe_encoder.transform(test_t)

{'cabin': {'B': 1.6299623810120747,
'C': 0.7217038208351837,
'D': 1.405081209799324,
'E': 1.405081209799324,
'Rare': 0.7387452866900354,
'n': -0.35752781962490193},
'pclass': {1: 0.9453018143294478,
2: 0.21009172435857942,
3: -0.5841726684724614},
'embarked': {'C': 0.6999054533737715,
'Q': -0.05044494288988759,
'S': -0.20113381737960143}}