API Reference

class feature_engine.encoding.RareLabelEncoder(tol=0.05, n_categories=10, max_n_categories=None, replace_with='Rare', variables=None, ignore_format=False)[source]

The RareLabelCategoricalEncoder() groups rare / infrequent categories in a new category called “Rare”, or any other name entered by the user.

For example in the variable colour, if the percentage of observations for the categories magenta, cyan and burgundy are < 5 %, all those categories will be replaced by the new label “Rare”.


Infrequent labels can also be grouped under a user defined name, for example ‘Other’. The name to replace infrequent categories is defined with the parameter replace_with.

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 finds the frequent labels for each variable (fit). The encoder then groups the infrequent labels under the new label ‘Rare’ or by another user defined string (transform).

tol: float, default=0.05

The minimum frequency a label should have to be considered frequent. Categories with frequencies lower than tol will be grouped.

n_categories: int, default=10

The minimum number of categories a variable should have for the encoder to find frequent labels. If the variable contains less categories, all of them will be considered frequent.

max_n_categories: int, default=None

The maximum number of categories that should be considered frequent. If None, all categories with frequency above the tolerance (tol) will be considered frequent. If you enter 5, only the 5 most frequent categories will be retained and the rest grouped.

replace_with: string, intege or float, default=’Rare’

The value that will be used to replace infrequent 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 frequent categories, i.e., those that will be kept, per variable.


The variables that will be transformed.


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



Find frequent categories.


Group rare categories


Fit to data, then transform it.

fit(X, y=None)[source]

Learn the frequent categories for each variable.

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

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

y: None

y is not required. You can pass y or None.

  • If the input is not a Pandas DataFrame.

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

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

  • If the variable(s) contain null values


If the number of categories in any one variable is less than the indicated in n_categories.


inverse_transform is not implemented for this transformer yet.


Group infrequent categories. Replace infrequent categories by the string ‘Rare’ or any other name provided by the user.

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

The input samples.

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

The dataframe where rare categories have been grouped.


DataFrame ..


If the input is not a Pandas DataFrame

  • If the variable(s) contain null values

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


The RareLabelEncoder() groups infrequent categories altogether into one new category called ‘Rare’ or a different string indicated by the user. We need to specify the minimum percentage of observations a category should show to be preserved and the minimum number of unique categories a variable should have to be re-grouped.

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 RareLabelEncoder

def load_titanic():
    data = pd.read_csv(
    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 the encoder
encoder = RareLabelEncoder(tol=0.03, n_categories=2, variables=['cabin', 'pclass', 'embarked'],

# fit the encoder

# transform the data
train_t = encoder.transform(X_train)
test_t = encoder.transform(X_test)

{'cabin': Index(['n', 'C', 'B', 'E', 'D'], dtype='object'),
 'pclass': array([2, 3, 1], dtype='int64'),
 'embarked': array(['S', 'C', 'Q'], dtype=object)}

You can also specify the maximum number of categories that can be considered frequent using the max_n_categories parameter.

from feature_engine.encoding import RareLabelEncoder
import pandas as pd
data = {'var_A': ['A'] * 10 + ['B'] * 10 + ['C'] * 2 + ['D'] * 1}
data = pd.DataFrame(data)
A    10
B    10
C     2
D     1
Name: var_A, dtype: int64
rare_encoder = RareLabelEncoder(tol=0.05, n_categories=3)
A       10
B       10
C        2
Rare     1
Name: var_A, dtype: int64
rare_encoder = RareLabelEncoder(tol=0.05, n_categories=3, max_n_categories=2)
Xt = rare_encoder.fit_transform(data)
A       10
B       10
Rare     3
Name: var_A, dtype: int64