Introduction: The following preprocessing steps may be applied to the data to help improve the accuracy, efficiency, and scalability of the classification or prediction process.

Data cleaning: This refers to the preprocessing of data in order to remove or reduce noise (by applying smoothing techniques, for example) and the treatment of missing values (e.g., by replacing a missing value with the most commonly occurring value for that attribute, or with the most probable value based on statistics). Although most classification algorithms have some mechanisms for handling noisy or missing data, this step can help reduce confusion during learning.

Relevance analysis: Many of the attributes in the data may be redundant. Correlation analysis can be used to identify whether any two given attributes are statistically related. For example, a strong correlation between attributes A1 and A2 would suggest that one of the two could be removed from further analysis. A database may also contain irrelevant attributes. Attribute subset selection4 can be used in these cases to find a reduced set of attributes such that the resulting probability distribution of the data classes is as close as possible to the original distribution obtained using all attributes. Hence, relevance analysis, in the form of correlation analysis and attribute subset selection, can be used to detect attributes that do not contribute to the classification or prediction task. Including such attributes may otherwise slow down, and possibly mislead, the learning step.

Ideally, the time spent on relevance analysis, when added to the time spent on learning from the resulting “reduced” attribute (or feature) subset, should be less than the time

That would have been spent on learning from the original set of attributes. Hence, such analysis can help improve classification efficiency and scalability.

Data transformation and reduction: The data may be transformed by normalization, particularly when neural networks or methods involving distance measurements are used in the learning step. Normalization involves scaling all values for a given attribute so that they fall within a small specified range, such as 1:0 to 1:0, or 0:0 to 1:0. In methods that use distance measurements, for example, this would prevent attributes with initially large ranges (like, say, income) from out weighing attributes with initially smaller ranges (such as binary attributes). The data can also be transformed by generalizing it to higher-level concepts. Concept hierarchies may be used for this purpose. This is particularly useful for continuous valued attributes. For example, numeric values for the attribute income can be generalized to discrete ranges, such as low, medium, and high. Similarly, categorical attributes, like street, can be generalized to higher-level concepts, like city. Because generalization compresses the original training data, fewer input/output operations may be involved during learning.

Data can also be reduced by applying many other methods, ranging from wavelet transformation and principle components analysis to discretization techniques, such as binning, histogram analysis, and clustering.