What is Unsupervised Machine Learning?

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Unsupervised Machine Learning trains models using unlabeled data. The model finds patterns or groups in the data by itself without human-provided labels.

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Clustering (a key unsupervised technique)

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Clustering groups unlabeled examples based on how similar they are to each other.

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• If the examples were labeled, this task would be called classification instead.

Single data point and notation

• A single data point (for example, a document) is written as $X_i$, where $i$ means the $i^{th}$ document or example.

Clusters and notation

• In clustering, clusters are unnamed (no human labels), so we usually number them.
• A cluster number is written as $Z_i$, where $i$ means the $i^{th}$ cluster.
• A cluster is defined by its center and its shape/spread.
• The center of a cluster is often called the centroid.

Human annotators