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1.5 Which Technologies Are Used?
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Unsupervised learning is essentially a synonym for clustering. The learning process
is unsupervised since the input examples are not class labeled. Typically, we may use
clustering to discover classes within the data. For example, an unsupervised learning
method can take, as input, a set of images of handwritten digits. Suppose that it finds
10 clusters of data. These clusters may correspond to the 10 distinct digits of 0 to
9, respectively. However, since the training data are not labeled, the learned model
cannot tell us the semantic meaning of the clusters found.
Semi-supervised learning is a class of machine learning techniques that make use
of both labeled and unlabeled examples when learning a model. In one approach,
labeled examples are used to learn class models and unlabeled examples are used to
refine the boundaries between classes. For a two-class problem, we can think of the
set of examples belonging to one class as the positive examples and those belonging
to the other class as the negative examples. In Figure 1.12, if we do not consider the
unlabeled examples, the dashed line is the decision boundary that best partitions
the positive examples from the negative examples. Using the unlabeled examples,
we can refine the decision boundary to the solid line. Moreover, we can detect that
the two positive examples at the top right corner, though labeled, are likely noise or
outliers.
Active learning is a machine learning approach that lets users play an active role
in the learning process. An active learning approach can ask a user (e.g., a domain
expert) to label an example, which may be from a set of unlabeled examples or
synthesized by the learning program. The goal is to optimize the model quality by
actively acquiring knowledge from human users, given a constraint on how many
examples they can be asked to label.
Positive example
Negative example
Unlabeled example
Decision boundary without unlabeled examples
Decision boundary with unlabeled examples
Noise/outliers
Figure 1.12
Semi-supervised learning.
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Chapter 1 Introduction
You can see there are many similarities between data mining and machine learning.
For classification and clustering tasks, machine learning research often focuses on the
accuracy of the model. In addition to accuracy, data mining research places strong
emphasis on the efficiency and scalability of mining methods on large data sets, as well
as on ways to handle complex types of data and explore new, alternative methods.
1.5.3
Database Systems and Data Warehouses
Database systems research focuses on the creation, maintenance, and use of databases
for organizations and end-users. Particularly, database systems researchers have estab-
lished highly recognized principles in data models, query languages, query processing
and optimization methods, data storage, and indexing and accessing methods. Database
systems are often well known for their high scalability in processing very large, relatively
structured data sets.
Many data mining tasks need to handle large data sets or even real-time, fast stream-
ing data. Therefore, data mining can make good use of scalable database technologies to
achieve high efficiency and scalability on large data sets. Moreover, data mining tasks can
be used to extend the capability of existing database systems to satisfy advanced users’
sophisticated data analysis requirements.
Recent database systems have built systematic data analysis capabilities on database
data using data warehousing and data mining facilities. A data warehouse integrates
data originating from multiple sources and various timeframes. It consolidates data
in multidimensional space to form partially materialized data cubes. The data cube
model not only facilitates OLAP in multidimensional databases but also promotes
multidimensional data mining (see Section 1.3.2).
1.5.4
Information Retrieval
Information retrieval (
IR) is the science of searching for documents or information
in documents. Documents can be text or multimedia, and may reside on the Web. The
differences between traditional information retrieval and database systems are twofold:
Information retrieval assumes that (1) the data under search are unstructured; and (2)
the queries are formed mainly by keywords, which do not have complex structures
(unlike SQL queries in database systems).
The typical approaches in information retrieval adopt probabilistic models. For
example, a text document can be regarded as a bag of words, that is, a multiset of words
appearing in the document. The document’s language model is the probability density
function that generates the bag of words in the document. The similarity between two
documents can be measured by the similarity between their corresponding language
models.
Furthermore, a topic in a set of text documents can be modeled as a probability dis-
tribution over the vocabulary, which is called a topic model. A text document, which
may involve one or multiple topics, can be regarded as a mixture of multiple topic mod-
els. By integrating information retrieval models and data mining techniques, we can find