Categorical data: Vocabulary and one-hot encoding

The term dimension is a synonym for the number of elements in a feature vector. Some categorical features are low dimensional. For example:

Feature name	# of categories	Sample categories
snowed_today	2	True, False
skill_level	3	Beginner, Practitioner, Expert
season	4	Winter, Spring, Summer, Autumn
day_of_week	7	Monday, Tuesday, Wednesday
planet	8	Mercury, Venus, Earth

When a categorical feature has a low number of possible categories, you can encode it as a vocabulary. With a vocabulary encoding, the model treats each possible categorical value as a separate feature. During training, the model learns different weights for each category.

For example, suppose you are creating a model to predict a car's price based, in part, on a categorical feature named car_color. Perhaps red cars are worth more than green cars. Since manufacturers offer a limited number of exterior colors, car_color is a low-dimensional categorical feature. The following illustration suggests a vocabulary (possible values) for car_color:

Figure 1. A unique feature for each category.

Exercise: Check your understanding

True or False: A machine learning model can train directly on raw string values, like "Red" and "Black", without converting these values to numerical vectors.

True

During training, a model can only manipulate floating-point numbers. The string "Red" is not a floating-point number. You must convert strings like "Red" to floating-point numbers.

False

A machine learning model can only train on features with floating-point values, so you'll need to convert those strings to floating-point values before training.

Index numbers

Machine learning models can only manipulate floating-point numbers. Therefore, you must convert each string to a unique index number, as in the following illustration:

Figure 2. Indexed features.

After converting strings to unique index numbers, you'll need to process the data further to represent it ways that help the model learn meaningful relationships between the values. If the categorical feature data is left as indexed integers and loaded into a model, the model would treat the indexed values as continuous floating-point numbers. The model would then consider "purple" six times more likely than "orange."

One-hot encoding

The next step in building a vocabulary is to convert each index number to its one-hot encoding. In a one-hot encoding:

• Each category is represented by a vector (array) of N elements, where N is the number of categories. For example, if car_color has eight possible categories, then the one-hot vector representing will have eight elements.
• Exactly one of the elements in a one-hot vector has the value 1.0; all the remaining elements have the value 0.0.

For example, the following table shows the one-hot encoding for each in car_color:

Feature	Red	Orange	Blue	Yellow	Green	Black	Purple	Brown
"Red"	1	0	0	0	0	0	0	0
"Orange"	0	1	0	0	0	0	0	0
"Blue"	0	0	1	0	0	0	0	0
"Yellow"	0	0	0	1	0	0	0	0
"Green"	0	0	0	0	1	0	0	0
"Black"	0	0	0	0	0	1	0	0
"Purple"	0	0	0	0	0	0	1	0
"Brown"	0	0	0	0	0	0	0	1

It is the one-hot vector, not the string or the index number, that gets passed to the feature vector. The model learns a separate weight for each element of the feature vector.

Note: In a true one-hot encoding, only one element has the value 1.0. In a variant known as multi-hot encoding, multiple values can be 1.0.

The following illustration suggests the various transformations in the vocabulary representation:

Figure 3. The end-to-end process to map categories to feature vectors.

Sparse representation

A feature whose values are predominately zero (or empty) is termed a sparse feature. Many categorical features, such as car_color, tend to be sparse features. Sparse representation means storing the position of the 1.0 in a sparse vector. For example, the one-hot vector for "Blue" is:

[0, 0, 1, 0, 0, 0, 0, 0]

Since the 1 is in position 2 (when starting the count at 0), the sparse representation for the preceding one-hot vector is:

2

Notice that the sparse representation consumes far less memory than the eight-element one-hot vector. Importantly, the model must train on the one-hot vector, not the sparse representation.

Note: The sparse representation of a multi-hot encoding stores the positions of all the nonzero elements. For example, the sparse representation of a car that is both "Blue" and "Black" is 2, 5.

Outliers in categorical data

Like numerical data, categorical data also contains outliers. Suppose car_color contains not only the popular colors, but also some rarely used outlier colors, such as "Mauve" or "Avocado". Rather than giving each of these outlier colors a separate category, you can lump them into a single "catch-all" category called out-of-vocabulary (OOV). In other words, all the outlier colors are binned into a single outlier bucket. The system learns a single weight for that outlier bucket.

Encoding high-dimensional categorical features

Some categorical features have a high number of dimensions, such as those in the following table:

Feature name	# of categories	Sample categories
words_in_english	~500,000	"happy", "walking"
US_postal_codes	~42,000	"02114", "90301"
last_names_in_Germany	~850,000	"Schmidt", "Schneider"

When the number of categories is high, one-hot encoding is usually a bad choice. Embeddings, detailed in a separate Embeddings module, are usually a much better choice. Embeddings substantially reduce the number of dimensions, which benefits models in two important ways:

• The model typically trains faster.
• The built model typically infers predictions more quickly. That is, the model has lower latency.

Hashing (also called the hashing trick) is a less common way to reduce the number of dimensions.

Click here to learn about hashing

In brief, hashing maps a category (for example, a color) to a small integer—the number of the "bucket" that will hold that category.

In detail, you implement a hashing algorithm as follows:

1. Set the number of bins in the vector of categories to N, where N is less than the total number of remaining categories. As an arbitrary example, say N = 100.
2. Choose a hash function. (Often, you will choose the range of hash values as well.)
3. Pass each category (for example, a particular color) through that hash function, generating a hash value, say 89237.
4. Assign each bin an index number of the output hash value modulo N. In this case, where N is 100 and the hash value is 89237, the modulo result is 37 because 89237 % 100 is 37.
5. Create a one-hot encoding for each bin with these new index numbers.

For more details about hashing data, see the Randomization section of the Production machine learning systems module.

Key terms:

• Categorical data
• Dimensions
• Discrete feature
• Feature cross
• Feature vector
• Hashing
• One-hot encoding
• Sparse feature
• Sparse representation