# Machine learning tasks in ML.NET

When building a machine learning model, you first need to define what you are hoping to achieve with your data. This allows you to choose the right machine learning task for your situation. The following list describes the different machine learning tasks that you can choose from and some common use cases. For more information about choosing the task that is appropriate for your scenario, see Algorithms.

Once you have decided which task works for your scenario, then you need to choose the best algorithm to train your model. The available algorithms are listed in the section for each task.

## Binary classification

A supervised machine learning task that is used to predict which of two classes (categories) an instance of data belongs to. The input of a classification algorithm is a set of labeled examples, where each label is an integer of either 0 or 1. The output of a binary classification algorithm is a classifier, which you can use to predict the class of new unlabeled instances. Examples of binary classification scenarios include:

• Understanding sentiment of Twitter comments as either "positive" or "negative".
• Diagnosing whether a patient has a certain disease or not.
• Making a decision to mark an email as "spam" or not.
• Determining if a photo contains a particular item or not, such as a dog or fruit.

For more information, see the Binary classification article on Wikipedia.

### Binary classification trainers

You can train a binary classification model using the following algorithms:

### Binary classification inputs and outputs

For best results with binary classification, the training data should be balanced (that is, equal numbers of positive and negative training data). Missing values should be handled before training.

The input label column data must be Boolean. The input features column data must be a fixed-size vector of Single.

These trainers output the following columns:

Output Column Name Column Type Description
Score Single The raw score that was calculated by the model
PredictedLabel Boolean The predicted label, based on the sign of the score. A negative score maps to false and a positive score maps to true.

## Multiclass classification

A supervised machine learning task that is used to predict the class (category) of an instance of data. The input of a classification algorithm is a set of labeled examples. Each label normally starts as text. It is then run through the TermTransform, which converts it to the Key (numeric) type. The output of a classification algorithm is a classifier, which you can use to predict the class of new unlabeled instances. Examples of multi-class classification scenarios include:

• Determining the breed of a dog as a "Siberian Husky", "Golden Retriever", "Poodle", etc.
• Understanding movie reviews as "positive", "neutral", or "negative".
• Categorizing hotel reviews as "location", "price", "cleanliness", etc.

For more information, see the Multiclass classification article on Wikipedia.

Note

One vs all upgrades any binary classification learner to act on multiclass datasets. More information on [Wikipedia] (https://en.wikipedia.org/wiki/Multiclass_classification#One-vs.-rest).

### Multiclass classification trainers

You can train a multiclass classification model using the following training algorithms:

### Multiclass classification inputs and outputs

The input label column data must be key type. The feature column must be a fixed size vector of Single.

This trainer outputs the following:

Output Name Type Description
Score Vector of Single The scores of all classes. Higher value means higher probability to fall into the associated class. If the i-th element has the largest value, the predicted label index would be i. Note that i is zero-based index.
PredictedLabel key type The predicted label's index. If its value is i, the actual label would be the i-th category in the key-valued input label type.

## Regression

A supervised machine learning task that is used to predict the value of the label from a set of related features. The label can be of any real value and is not from a finite set of values as in classification tasks. Regression algorithms model the dependency of the label on its related features to determine how the label will change as the values of the features are varied. The input of a regression algorithm is a set of examples with labels of known values. The output of a regression algorithm is a function, which you can use to predict the label value for any new set of input features. Examples of regression scenarios include:

• Predicting house prices based on house attributes such as number of bedrooms, location, or size.
• Predicting future stock prices based on historical data and current market trends.
• Predicting sales of a product based on advertising budgets.

### Regression trainers

You can train a regression model using the following algorithms:

### Regression inputs and outputs

The input label column data must be Single.

The trainers for this task output the following:

Output Name Type Description
Score Single The raw score that was predicted by the model

## Clustering

An unsupervised machine learning task that is used to group instances of data into clusters that contain similar characteristics. Clustering can also be used to identify relationships in a dataset that you might not logically derive by browsing or simple observation. The inputs and outputs of a clustering algorithm depends on the methodology chosen. You can take a distribution, centroid, connectivity, or density-based approach. ML.NET currently supports a centroid-based approach using K-Means clustering. Examples of clustering scenarios include:

• Understanding segments of hotel guests based on habits and characteristics of hotel choices.
• Identifying customer segments and demographics to help build targeted advertising campaigns.
• Categorizing inventory based on manufacturing metrics.

### Clustering trainer

You can train a clustering model using the following algorithm:

### Clustering inputs and outputs

The input features data must be Single. No labels are needed.

This trainer outputs the following:

Output Name Type Description
Score vector of Single The distances of the given data point to all clusters' centriods
PredictedLabel key type The closest cluster's index predicted by the model.

## Anomaly detection

This task creates an anomaly detection model by using Principal Component Analysis (PCA). PCA-Based Anomaly Detection helps you build a model in scenarios where it is easy to obtain training data from one class, such as valid transactions, but difficult to obtain sufficient samples of the targeted anomalies.

An established technique in machine learning, PCA is frequently used in exploratory data analysis because it reveals the inner structure of the data and explains the variance in the data. PCA works by analyzing data that contains multiple variables. It looks for correlations among the variables and determines the combination of values that best captures differences in outcomes. These combined feature values are used to create a more compact feature space called the principal components.

Anomaly detection encompasses many important tasks in machine learning:

• Identifying transactions that are potentially fraudulent.
• Learning patterns that indicate that a network intrusion has occurred.
• Finding abnormal clusters of patients.
• Checking values entered into a system.

Because anomalies are rare events by definition, it can be difficult to collect a representative sample of data to use for modeling. The algorithms included in this category have been especially designed to address the core challenges of building and training models by using imbalanced data sets.

### Anomaly detection trainer

You can train an anomaly detection model using the following algorithm:

### Anomaly detection inputs and outputs

The input features must be a fixed-sized vector of Single.

This trainer outputs the following:

Output Name Type Description
Score Single The non-negative, unbounded score that was calculated by the anomaly detection model

## Ranking

A ranking task constructs a ranker from a set of labeled examples. This example set consists of instance groups that can be scored with a given criteria. The ranking labels are { 0, 1, 2, 3, 4 } for each instance. The ranker is trained to rank new instance groups with unknown scores for each instance. ML.NET ranking learners are machine learned ranking based.

### Ranking training algorithms

You can train a ranking model with the following algorithms:

### Ranking input and outputs

The input label data type must be key type or Single. The value of the label determines relevance, where higher values indicate higher relevance. If the label is a key type, then the key index is the relevance value, where the smallest index is the least relevant. If the label is a Single, larger values indicate higher relevance.

The feature data must be a fixed size vector of Single and input row group column must be key type.

This trainer outputs the following:

Output Name Type Description
Score Single The unbounded score that was calculated by the model to determine the prediction

## Recommendation

A recommendation task enables producing a list of recommended products or services. ML.NET uses Matrix factorization (MF), a collaborative filtering algorithm for recommendations when you have historical product rating data in your catalog. For example, you have historical movie rating data for your users and want to recommend other movies they are likely to watch next.

### Recommendation training algorithms

You can train a recommendation model with the following algorithm: