CombineWithReferenceFeature() combines a group of variables with a group of reference variables utilizing basic mathematical operations (subtraction, division, addition and multiplication). It returns one or more additional features in the dataframe as a result of these operations.

In other words, CombineWithReferenceFeature() sums, multiplies, subtracts or divides a group of features (indicated in variables_to_combine) to or by a group of reference variables (indicated in reference_variables), and returns the result as new variables in the dataframe.

For example, if we have the variables:

  • number_payments_first_quarter,

  • number_payments_second_quarter,

  • number_payments_third_quarter,

  • number_payments_fourth_quarter, and

  • total_payments,

we can use CombineWithReferenceFeature() to determine the percentage of payments per quarter as follows:

transformer = CombineWithReferenceFeature(




Xt = transformer.fit_transform(X)

The precedent code block will return a new dataframe, Xt, with 4 new variables, those indicated in new_variables_name, that are calculated as the division of each one of the variables in variables_to_combine and ‘total_payments’.

Below we show another example using the House Prices Dataset (more details about the dataset here). In this example, we subtract LotFrontage from LotArea.

import pandas as pd
from sklearn.model_selection import train_test_split

from feature_engine.creation import CombineWithReferenceFeature

data = pd.read_csv('houseprice.csv').fillna(0)

X_train, X_test, y_train, y_test = train_test_split(
data.drop(['Id', 'SalePrice'], axis=1),

combinator = CombineWithReferenceFeature(
    operations = ['sub'],
    new_variables_names = ['LotPartial']
    ), y_train)

X_train = combinator.transform(X_train)

We can see the newly created variable in the following code blocks:

    LotTotal  LotFrontage  LotArea
64      9375.0          0.0     9375
682     2887.0          0.0     2887
960     7157.0         50.0     7207
1384    9000.0         60.0     9060
1100    8340.0         60.0     8400


Even though the transfomer allows to combine variables automatically, it was originally designed to combine variables with domain knowledge. In this case, we normally want to give meaningful names to the variables. We can do so through the parameter new_variables_names.

new_variables_names takes a list of strings, with the new variable names. In this parameter, you need to enter as many names as new features are created by the transformer. The number of new features is the number of operations, times the number of reference variables, times the number of variables to combine.

Thus, if you want to perform 2 operations, sub and div, combining 4 variables with 2 reference variables, you should enter 2 X 4 X 2 new variable names.

The name of the variables should coincide with the order in which the operations are performed by the transformer. The transformer will first carry out ‘sub’, then ‘div’, then ‘add’ and finally ‘mul’.

More details#

You can find creative ways to use the CombineWithReferenceFeature() in the following Jupyter notebooks and Kaggle kernels.