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- 2. Data Preprocessing Why preprocess the data? Data cleaning Data integration and transformation Data reduction Discretization and concept hierarchy generation Summary
- 3. Why Data Preprocessing? Data in the real world is dirty incomplete: lacking attribute values, lacking certain attributes of interest, or containing only aggregate data noisy: containing errors or outliers inconsistent: containing discrepancies in codes or names No quality data, no quality mining results! Quality decisions must be based on quality data Data warehouse needs consistent integration of quality data
- 4. Multi-Dimensional Measure of Data Quality A well-accepted multidimensional view: Accuracy Completeness Consistency Timeliness Believability Value added Interpretability Accessibility
- 5. Major Tasks in Data Preprocessing Data cleaning Fill in missing values, smooth noisy data, identify or remove outliers, and resolve inconsistencies Data integration Integration of multiple databases, data cubes, or files Data transformation Normalization and aggregation Data reduction Obtains reduced representation in volume but produces the same or similar analytical results Data discretization Part of data reduction but with particular importance, especially for numerical data
- 7. Data Preprocessing Why preprocess the data? Data cleaning Data integration and transformation Data reduction Discretization and concept hierarchy generation Summary
- 8. Data Cleaning Data cleaning tasks Fill in missing values Identify outliers and smooth out noisy data Correct inconsistent data
- 9. Missing Data Data is not always available E.g., many tuples have no recorded value for several attributes, such as customer income in sales data Missing data may be due to equipment malfunction inconsistent with other recorded data and thus deleted data not entered due to misunderstanding certain data may not be considered important at the time of entry not register history or changes of the data Missing data may need to be inferred.
- 10. How to Handle Missing Data? Ignore the tuple: usually done when class label is missing (assuming the tasks in classification—not effective when the percentage of missing values per attribute varies considerably) Fill in the missing value manually: tedious + infeasible? Use a global constant to fill in the missing value: e.g., “unknown”, a new class?! Use the attribute mean to fill in the missing value Use the most probable value to fill in the missing value: inference- based such as Bayesian formula or decision tree
- 11. Noisy Data Noise: random error or variance in a measured variable Incorrect attribute values may due to faulty data collection instruments data entry problems data transmission problems technology limitation inconsistency in naming convention Other data problems which requires data cleaning duplicate records incomplete data inconsistent data
- 12. How to Handle Noisy Data? Binning method: first sort data and partition into (equi-depth) bins then smooth by bin means, smooth by bin median, smooth by bin boundaries, etc. Clustering detect and remove outliers Combined computer and human inspection detect suspicious values and check by human Regression smooth by fitting the data into regression functions
- 13. Simple Discretization Methods: Binning Equal-width (distance) partitioning: It divides the range into N intervals of equal size: uniform grid if A and B are the lowest and highest values of the attribute, the width of intervals will be: W = (B-A)/N. The most straightforward But outliers may dominate presentation Skewed data is not handled well. Equal-depth (frequency) partitioning: It divides the range into N intervals, each containing approximately same number of samples Good data scaling Managing categorical attributes can be tricky.
- 14. Binning Methods for Data Smoothing * Sorted data for price (in dollars): 4, 8, 9, 15, 21, 21, 24, 25, 26, 28, 29, 34 * Partition into (equi-depth) bins: - Bin 1: 4, 8, 9, 15 - Bin 2: 21, 21, 24, 25 - Bin 3: 26, 28, 29, 34 * Smoothing by bin means: - Bin 1: 9, 9, 9, 9 - Bin 2: 23, 23, 23, 23 - Bin 3: 29, 29, 29, 29 * Smoothing by bin boundaries: - Bin 1: 4, 4, 4, 15 - Bin 2: 21, 21, 25, 25 - Bin 3: 26, 26, 26, 34
- 15. Data Preprocessing Why preprocess the data? Data cleaning Data integration and transformation Data reduction Discretization and concept hierarchy generation Summary
- 16. Data Integration Data integration: combines data from multiple sources into a coherent store Schema integration integrate metadata from different sources Entity identification problem: identify real world entities from multiple data sources, e.g., A.cust-id ≡ B.cust-# Detecting and resolving data value conflicts for the same real world entity, attribute values from different sources are different possible reasons: different representations, different scales, e.g., metric vs. British units
- 17. Handling Redundant Data Redundant data occur often when integration of multiple databases The same attribute may have different names in different databasesCareful integration of the data from multiple sources may help reduce/avoid redundancies and inconsistencies and improve mining speed and quality
- 18. Data Transformation Smoothing: remove noise from data Aggregation: summarization, data cube construction Generalization: concept hierarchy climbing Normalization: scaled to fall within a small, specified range min-max normalization z-score normalization normalization by decimal scaling
- 19. Data Transformation: Normalization min-max normalization z-score normalization normalization by decimal scaling A A A A A A min new min new max new min max min v v _ ) _ _ ( ' + − − − = A A dev stand mean v v _ ' − = j v v 10 '= Where j is the smallest integer such that Max(| |)<1 ' v
- 20. Data Preprocessing Why preprocess the data? Data cleaning Data integration and transformation Data reduction Discretization and concept hierarchy generation Summary
- 21. Data Reduction Strategies Warehouse may store terabytes of data: Complex data analysis/mining may take a very long time to run on the complete data set Data reduction Obtains a reduced representation of the data set that is much smaller in volume but yet produces the same (or almost the same) analytical results Data reduction strategies Data cube aggregation Dimensionality reduction Numerosity reduction Discretization and concept hierarchy generation
- 22. Data Cube Aggregation The lowest level of a data cube the aggregated data for an individual entity of interest e.g., a customer in a phone calling data warehouse. Multiple levels of aggregation in data cubes Further reduce the size of data to deal with Reference appropriate levels Use the smallest representation which is enough to solve the task
- 23. Dimensionality Reduction Feature selection (i.e., attribute subset selection): Select a minimum set of features such that the probability distribution of different classes given the values for those features is as close as possible to the original distribution given the values of all features reduce # of patterns in the patterns, easier to understand
- 24. Example of Decision Tree Induction Initial attribute set: {A1, A2, A3, A4, A5, A6} A4 ? A1? A6? Class 1 Class 2 Class 1 Class 2 > Reduced attribute set: {A1, A4, A6}
- 25. Regression and Log- Linear Models Linear regression: Data are modeled to fit a straight line Often uses the least-square method to fit the line Multiple regression: allows a response variable Y to be modeled as a linear function of multidimensional feature vector Log-linear model: approximates discrete multidimensional probability distributions
- 26. Linear regression: Y = α + β X Two parameters , α and β specify the line and are to be estimated by using the data at hand. using the least squares criterion to the known values of Y1, Y2, …, X1, X2, …. Multiple regression: Y = b0 + b1 X1 + b2 X2. Many nonlinear functions can be transformed into the above. Log-linear models: The multi-way table of joint probabilities is approximated by a product of lower-order tables. Probability: p(a, b, c, d) = αab βacχad δbcd Regress Analysis and Log-Linear Models
- 27. Histograms A popular data reduction technique Divide data into buckets and store average (sum) for each bucket Can be constructed optimally in one dimension using dynamic programming Related to quantization problems. 0 5 10 15 20 25 30 35 40 10000 20000 30000 40000 50000 60000 70000 80000 90000 100000
- 28. Clustering Partition data set into clusters, and one can store cluster representation only Can be very effective if data is clustered but not if data is “smeared” Can have hierarchical clustering and be stored in multi- dimensional index tree structures There are many choices of clustering definitions and clustering algorithms, further detailed in Chapter 8
- 29. Sampling Allow a mining algorithm to run in complexity that is potentially sub-linear to the size of the data Choose a representative subset of the data Simple random sampling may have very poor performance in the presence of skew Develop adaptive sampling methods Stratified sampling: Approximate the percentage of each class (or subpopulation of interest) in the overall database Used in conjunction with skewed data
- 31. Data Preprocessing Why preprocess the data? Data cleaning Data integration and transformation Data reduction Discretization and concept hierarchy generation Summary
- 32. Discretization Three types of attributes: Nominal — values from an unordered set Ordinal — values from an ordered set Continuous — real numbers Discretization: divide the range of a continuous attribute into intervals Some classification algorithms only accept categorical attributes. Reduce data size by discretization Prepare for further analysis
- 33. Discretization and Concept hierachy Discretization reduce the number of values for a given continuous attribute by dividing the range of the attribute into intervals. Interval labels can then be used to replace actual data values. Concept hierarchies reduce the data by collecting and replacing low level concepts (such as numeric values for the attribute age) by higher level concepts (such as young, middle-aged, or senior).
- 34. Discretization for numeric data Binning (see sections before) Histogram analysis (see sections before) Clustering analysis (see sections before)
- 35. Data Preprocessing Why preprocess the data? Data cleaning Data integration and transformation Data reduction Discretization and concept hierarchy generation Summary
- 36. Summary Data preparation is a big issue for both warehousing and mining Data preparation includes Data cleaning and data integration Data reduction and feature selection Discretization A lot a methods have been developed but still an active area of research
- 37. References D. P. Ballou and G. K. Tayi. Enhancing data quality in data warehouse environments. Communications of ACM, 42:73-78, 1999. Jagadish et al., Special Issue on Data Reduction Techniques. Bulletin of the Technical Committee on Data Engineering, 20(4), December 1997. D. Pyle. Data Preparation for Data Mining. Morgan Kaufmann, 1999. T. Redman. Data Quality: Management and Technology. Bantam Books, New York, 1992. Y. Wand and R. Wang. Anchoring data quality dimensions ontological foundations. Communications of ACM, 39:86-95, 1996. R. Wang, V. Storey, and C. Firth. A framework for analysis of data quality research. IEEE Trans. Knowledge and Data Engineering, 7:623-640, 1995.