ArbitraryDiscretiser#
The ArbitraryDiscretiser() sorts the variable values into contiguous intervals
which limits are arbitrarily defined by the user. Thus, you must provide a dictionary
with the variable names as keys and the limits of the intervals in a list as values,
when setting up the discretiser.
The ArbitraryDiscretiser() works only with numerical variables. The discretiser
will check that the variables entered by the user are present in the train set and cast
as numerical.
Example
Let’s take a look at how this transformer works. First, let’s load a dataset and plot a histogram of a continuous variable. We use the boston house prices dataset that comes with Scikit-learn.
import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
from sklearn.datasets import load_boston
from feature_engine.discretisation import ArbitraryDiscretiser
boston_dataset = load_boston()
data = pd.DataFrame(boston_dataset.data, columns=boston_dataset.feature_names)
data['LSTAT'].hist(bins=20)
plt.xlabel('LSTAT')
plt.ylabel('Number of observations')
plt.title('Histogram of LSTAT')
plt.show()
Now, let’s discretise the variable into arbitrarily determined intervals. We want the
interval names as integers, so we set return_boundaries to False.
user_dict = {'LSTAT': [0, 10, 20, 30, np.Inf]}
transformer = ArbitraryDiscretiser(
binning_dict=user_dict, return_object=False, return_boundaries=False)
X = transformer.fit_transform(data)
Now, we can go ahead and plot the variable after the transformation:
X['LSTAT'].value_counts().plot.bar()
plt.xlabel('LSTAT - bins')
plt.ylabel('Number of observations')
plt.title('Discretised LSTAT')
plt.show()
Note that in the above figure the intervals are represented by digits.
Alternatively, we can return the interval limits in the discretised variable by
setting return_boundaries to True.
transformer = ArbitraryDiscretiser(
binning_dict=user_dict, return_object=False, return_boundaries=True)
X = transformer.fit_transform(data)
X['LSTAT'].value_counts().plot.bar(rot=0)
plt.xlabel('LSTAT - bins')
plt.ylabel('Number of observations')
plt.title('Discretised LSTAT')
plt.show()
Discretisation plus encoding
If we return the interval values as integers, the discretiser has the option to return the transformed variable as integer or as object. Why would we want the transformed variables as object?
Categorical encoders in Feature-engine are designed to work with variables of type
object by default. Thus, if you wish to encode the returned bins further, say to try and
obtain monotonic relationships between the variable and the target, you can do so
seamlessly by setting return_object to True. You can find an example of how to use
this functionality here.