The RareLabelEncoder() groups infrequent categories 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 have to be preserved and the minimum number of unique categories a variable should have to be re-grouped.


In the parameter tol we indicate the minimum proportion of observations a category should have, not to be grouped. In other words, categories which frequency, or proportion of observations is <= tol will be grouped into a unique term.


In the parameter n_categories we indicate the minimum cardinality of the categorical variable in order to group infrequent categories. For example, if n_categories=5, categories will be grouped only in those categorical variables with more than 5 unique categories. The rest of the variables will be ignored.

This parameter is useful when we have big datasets and do not have time to examine all categorical variables individually. This way, we ensure that variables with low cardinality are not reduced any further.


In the parameter max_n_categories we indicate the maximum number of unique categories that we want in the encoded variable. If max_n_categories=5, then the most popular 5 categories will remain in the variable after the encoding, all other will be grouped into a single category.

This parameter is useful if we are going to perform one hot encoding at the back of it, to control the expansion of the feature space.


Let’s look at an example using the Titanic Dataset.

First, let’s load the data and separate it into train and test:

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)

Now, we set up the RareLabelEncoder() to group categories shown by less than 3% of the observations into a new group or category called ‘Rare’. We will group the categories in the indicated variables if they have more than 2 unique categories each.

# set up the encoder
encoder = RareLabelEncoder(tol=0.03, n_categories=2, variables=['cabin', 'pclass', 'embarked'],

# fit the encoder

With fit(), the RareLabelEncoder() finds the categories present in more than 3% of the observations, that is, those that will not be grouped. These categories can be found in the encoder_dict_ attribute.


In the encoder_dict_ we find the most frequent categories per variable to encode. Any category that is not in this dictionary, will be grouped.

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

Now we can go ahead and transform the variables:

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

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

Let’s begin by creating a toy dataframe and count the values of observations per category:

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

In this block of code, we group the categories only for variables with more than 3 unique categories and then we plot the result:

rare_encoder = RareLabelEncoder(tol=0.05, n_categories=3)
A       10
B       10
C        2
Rare     1
Name: var_A, dtype: int64

Now, we retain the 2 most frequent categories of the variable and group the rest into the ‘Rare’ group:

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


The RareLabelEncoder() can be used to group infrequent categories and like this control the expansion of the feature space if using one hot encoding.

Some categorical encodings will also return NAN if a category is present in the test set, but was not seen in the train set. This inconvenient can usually be avoided if we group rare labels before training the encoders.

Some categorical encoders will also return NAN if there is not enough observations for a certain category. For example the WoEEncoder() and the PRatioEncoder(). This behaviour can be also prevented by grouping infrequent labels before the encoding with the RareLabelEncoder().

More details#

In the following notebook, you can find more details into the RareLabelEncoder() functionality and example plots with the encoded variables:

All notebooks can be found in a dedicated repository.