Dealing with categorical data

Friday, January 19, 2018
3 mins read

Often in our machine learning model, we encounter qualitative predictors.

One of the ways to deal with these factors is to create a dummy variable. If a factor has two levels or possible values, then we simply create a dummy variable that takes two possible numerical values. For example, if gender variable takes two values – Male and Female.

and include this new variable in our model. This results in the model:

If a qualitative predictor has more than two levels, we create additional dummy variables. For example, if quality has three possible values – Bad, Medium, and Good.

We deal with these factors in the following ways:

One Hot encoding and dummy encoding

One hot encoding is one of the widespread method used to deal with the categorical values. Another method is dummy encoding. There is a slight difference between the two.

For example, if one categorical variable has n values. One-hot encoding converts it into n variables, while dummy encoding converts it into n-1 variables.

Using pandas

PyDev console: using IPython 6.1.0
Python 3.6.3 |Anaconda, Inc.| (default, Nov  3 2017, 19:19:16) 
[GCC 7.2.0] on linux
In[2]: import pandas as pd
In[3]: df = pd.DataFrame({'quality': ['bad', 'medium', 'good']})
In[4]: df
Out[4]: 
  quality
0     bad
1  medium
2    good
In[5]: df.info()
<class'pandas.core.frame.DataFrame'>
RangeIndex: 3 entries, 0 to 2
Data columns (total 1 columns):
quality    3 non-null object
dtypes: object(1)
memory usage: 104.0+ bytes
In[6]: df = pd.get_dummies(df)  # one hot encoding
In[7]: df
Out[7]: 
   quality_bad  quality_good  quality_medium
0            1             0               0
1            0             0               1
2            0             1               0

By default, python’s get_dummies doesn’t do dummy encoding but one-hot encoding. To produce dummy encoding, use drop_first=True.

In[8]: df = pd.get_dummies(df, drop_first=True)  # dummy encoding
In[9]: df
Out[9]: 
   quality_good  quality_medium
0             0               0
1             0               1
2             1               0

Using scikit-learn

scikit-learn also provides methods to deal with categorical variables e.g. sklearn.preprocessing.LabelEncoder(). LabelEncoder is incremental encoding, such as 0,1,2,3,4,… We can also use scikit-learn’s sklearn.preprocessing.OneHotEncoder(). There is one more type of encoding, Frequency encoding, which maps categories to their frequencies.

Label and Frequency encodings are often used for tree based methods while one hot encoding is used for non-tree-based models.

References:

  1. pandas.get_dummies
  2. sklearn.preprocessing.OneHotEncoder

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