This document discusses various techniques for data preprocessing including data cleaning, integration, transformation, reduction, discretization, and concept hierarchy generation. Data cleaning involves handling missing data, noisy data, and inconsistent data through techniques like filling in missing values, identifying outliers, and correcting errors. Data integration combines data from multiple sources and resolves issues like redundant or conflicting data. Data transformation techniques normalize data scales and construct new attributes. Data reduction methods like sampling, clustering, and histograms reduce data volume while maintaining analytical quality. Discretization converts continuous attributes to categorical bins. Concept hierarchies generalize data by grouping values into higher-level concepts.