Data Mining by Mehmed Kantardzic (inspirational novels TXT) 📗

2. Predictive/Descriptive Accuracy. This is the dominant measure for most data-mining models since it measures how well the data are summarized and generalized into the model. We generally expect that by using only relevant features, a data-mining algorithm can learn not only faster but also with higher accuracy. Irrelevant data may mislead a learning process and a final model, while redundant data may complicate the task of learning and cause unexpected data-mining results.

3. Representation of the Data-Mining Model. The simplicity of representation, obtained usually with data reduction, often implies that a model can be better understood. The simplicity of the induced model and other results depends on its representation. Therefore, if the simplicity of representation improves, a relatively small decrease in accuracy may be tolerable. The need for a balanced view between accuracy and simplicity is necessary, and dimensionality reduction is one of the mechanisms for obtaining this balance.

It would be ideal if we could achieve reduced time, improved accuracy, and simplified representation at the same time, using dimensionality reduction. More often than not, however, we gain in some and lose in others, and balance between them according to the application at hand. It is well known that no single data-reduction method can be best suited for all applications. A decision about method selection is based on available knowledge about an application (relevant data, noise data, meta-data, correlated features), and required time constraints for the final solution.

Algorithms that perform all basic operations for data reduction are not simple, especially when they are applied to large data sets. Therefore, it is useful to enumerate the desired properties of these algorithms before giving their detailed descriptions. Recommended characteristics of data-reduction algorithms that may be guidelines for designers of these techniques are as follows:

1. Measurable Quality. The quality of approximated results using a reduced data set can be determined precisely.

2. Recognizable Quality. The quality of approximated results can be easily determined at run time of the data-reduction algorithm, before application of any data-mining procedure.

3. Monotonicity. The algorithms are usually iterative, and the quality of results is a nondecreasing function of time and input data quality.

4. Consistency. The quality of results is correlated with computation time and input data quality.

5. Diminishing Returns. The improvement in the solution is large in the early stages (iterations) of the computation, and it diminishes over time.

6. Interruptability. The algorithm can be stopped at any time and provide some answers.

7. Preemptability. The algorithm can be suspended and resumed with minimal overhead.

3.2 FEATURE REDUCTION

Most of the real-world data mining applications are characterized by high-dimensional data, where not all of the features are important. For example, high-dimensional data (i.e., data sets with hundreds or even thousands of features) can contain a lot of irrelevant, noisy information that may greatly degrade the performance of a data-mining process. Even state-of-the-art data-mining algorithms cannot overcome the presence of a large number of weakly relevant and redundant features. This is usually attributed to the “curse of dimensionality,” or to the fact that irrelevant features decrease the signal-to-noise ratio. In addition, many algorithms become computationally intractable when the dimensionality is high.

Data such as images, text, and multimedia are high-dimensional in nature, and this dimensionality of data poses a challenge to data-mining tasks. Researchers have found that reducing the dimensionality of data results in a faster computation while maintaining reasonable accuracy. In the presence of many irrelevant features, mining algorithms tend to overfit the model. Therefore, many features can be removed without performance deterioration in the mining process.

When we are talking about data quality and improved performances of reduced data sets, we can see that this issue is not only about noisy or contaminated data (problems mainly solved in the preprocessing phase), but also about irrelevant, correlated, and redundant data. Recall that data with corresponding features are not usually collected solely for data-mining purposes. Therefore, dealing with relevant features alone can be far more effective and efficient. Basically, we want to choose features that are relevant to our data-mining application in order to achieve maximum performance with minimum measurement and processing effort. A feature-reduction process should result in

1. less data so that the data-mining algorithm can learn faster;

2. higher accuracy of a data-mining process so that the model can generalize better from the data;

3. simple results of the data-mining process so that they are easier to understand and use; and

4. fewer features so that in the next round of data collection, savings can be made by removing redundant or irrelevant features.

Let us start our detailed analysis of possible column-reduction techniques, where features are eliminated from the data set based on a given criterion. To address the curse of dimensionality, dimensionality-reduction techniques are proposed as a data-preprocessing step. This process identifies a suitable low-dimensional representation of original data. Reducing the dimensionality improves the computational efficiency and accuracy of the data analysis. Also, it improves comprehensibility of a data-mining model. Proposed techniques are classified as supervised and unsupervised techniques based on the type of learning process. Supervised algorithms need a training set with the output class label information to learn the lower dimensional representation according to some criteria. Unsupervised approaches project the original data to a new lower dimensional space without utilizing the label (class) information. Dimensionality-reduction techniques function either by transforming the existing features to a new reduced set of features or by selecting a subset of the existing features. Therefore, two standard tasks are associated with producing a reduced set of features, and they are classified as:

1. Feature Selection. Based on the knowledge of the application domain and the goals of the mining effort, the human analyst may select a subset of the features found in the initial data set. The process of feature selection can be manual or supported by some automated procedures.

Roughly speaking, feature-selection methods are applied in one of three conceptual frameworks: the filter model, the wrapper model, and embedded methods. These three basic families