The Importance of Data Preprocessing in Data Analysis

The Importance of Data Preprocessing in Data Analysis

This article explains the significance of data preprocessing in preparing data for analysis. Topics covered include data cleaning, integration and transformation, data reduction, and concept hierarchy generation.

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1. 1 DATA PREPROCESSING Why preprocess the data? Descriptive data summarization Data cleaning Data integration and transformation Data reduction Discretization and concept hierarchy generation Summary

2. April 13, 2015 2 Why Data Preprocessing? Data in the real world is dirty incomplete : lacking attribute values, lacking certain attributes of interest, or containing only aggregate data e.g., occupation= noisy : containing errors or outliers e.g., Salary=-10 inconsistent : containing discrepancies in codes or names e.g., Age=42 Birthday=03/07/1997 e.g., Was rating 1,2,3, now rating A, B, C e.g., discrepancy between duplicate records

3. April 13, 2015 3 Why Is Data Dirty? Incomplete data may come from Not applicable data value when collected Different considerations between the time when the data was collected and when it is analyzed. Human/hardware/software problems Noisy data (incorrect values) may come from Faulty data collection instruments Human or computer error at data entry Errors in data transmission Inconsistent data may come from Different data sources Functional dependency violation (e.g., modify some linked data) Duplicate records also need data cleaning

4. April 13, 2015 4 Why Is Data Preprocessing Important? No quality data, no quality mining results! Quality decisions must be based on quality data e.g., duplicate or missing data may cause incorrect or even misleading statistics. Data warehouse needs consistent integration of quality data Data extraction, cleaning, and transformation comprises the majority of the work of building a data warehouse

5. April 13, 2015 5 Multi-Dimensional Measure of Data Quality A well-accepted multidimensional view: Accuracy Completeness Consistency Timeliness Believability Value added Interpretability Accessibility Broad categories: Intrinsic, contextual, representational, and accessibility

6. April 13, 2015 6 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. April 13, 2015 7 Forms of Data Preprocessing

8. April 13, 2015 8 Chapter 2: Data Preprocessing Why preprocess the data? Descriptive data summarization Data cleaning Data integration and transformation Data reduction Discretization and concept hierarchy generation Summary

9. April 13, 2015 9 Mining Data Descriptive Characteristics Motivation To better understand the data: central tendency, variation and spread Data dispersion characteristics median, max, min, quantiles, outliers, variance, etc. Numerical dimensions correspond to sorted intervals Data dispersion: analyzed with multiple granularities of precision Boxplot or quantile analysis on sorted intervals Dispersion analysis on computed measures Folding measures into numerical dimensions Boxplot or quantile analysis on the transformed cube

10. April 13, 2015 10 Measuring the Central Tendency Mean (algebraic measure) (sample vs. population): Weighted arithmetic mean: Trimmed mean: chopping extreme values Median : A holistic measure Middle value if odd number of values, or average of the middle two values otherwise Estimated by interpolation (for grouped data ): Mode Value that occurs most frequently in the data Unimodal, bimodal, trimodal Empirical formula:

11. April 13, 2015 11 Symmetric vs. Skewed Data Median, mean and mode of symmetric, positively and negatively skewed data

12. April 13, 2015 12 Measuring the Dispersion of Data Quartiles, outliers and boxplots Quartiles : Q 1 (25 th percentile), Q 3 (75 th percentile) Inter-quartile range : IQR = Q 3 Q 1 Five number summary : min, Q 1 , M, Q 3 , max Boxplot : ends of the box are the quartiles, median is marked, whiskers, and plot outlier individually Outlier : usually, a value higher/lower than 1.5 x IQR Variance and standard deviation ( sample: s, population: ) Variance : (algebraic, scalable computation) Standard deviation s (or ) is the square root of variance s 2 ( or 2)

13. April 13, 2015 13 Properties of Normal Distribution Curve The normal (distribution) curve From to + : contains about 68% of the measurements ( : mean, : standard deviation) From 2 to +2 : contains about 95% of it From 3 to +3 : contains about 99.7% of it

14. April 13, 2015 14 Boxplot Analysis Five-number summary of a distribution: Minimum, Q1, M, Q3, Maximum Boxplot Data is represented with a box The ends of the box are at the first and third quartiles, i.e., the height of the box is IRQ The median is marked by a line within the box Whiskers: two lines outside the box extend to Minimum and Maximum

15. April 13, 2015 15 Visualization of Data Dispersion: Boxplot Analysis

16. April 13, 2015 16 Data Preprocessing Why preprocess the data? Descriptive data summarization Data cleaning Data integration and transformation Data reduction Discretization and concept hierarchy generation Summary

17. April 13, 2015 17 Data Cleaning Importance Data cleaning is one of the three biggest problems in data warehousingRalph Kimball Data cleaning is the number one problem in data warehousing survey Data cleaning tasks Fill in missing values Identify outliers and smooth out noisy data Correct inconsistent data Resolve redundancy caused by data integration

18. April 13, 2015 18 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.

19. April 13, 2015 19 Data Mining Cup 2004 425 students from 166 universities and 32 countries took part in the competition, which lasted from April 15, 2004 to May 13, 2004. 111 participants submitted solution models. The objective of data mining is to discover hidden relations, patterns, and trends in databases. This year's data mining task dealt with the issue of predicting the behavior of customers returning mail-order merchandise. Yuchun Tang's solution ranked 50th with 9559 points (the top-ranked solution received 10511 points) Winner' presentation DMC 2004 (PDF)

20. April 13, 2015 20 How to Handle Missing Data? Ignore the tuple: usually done when class label is missing (assuming the tasks in classificationnot effective when the percentage of missing values per attribute varies considerably. Fill in the missing value manually: tedious + infeasible? Fill in it automatically with a global constant : e.g., unknown, a new class?! the attribute mean the attribute mean for all samples belonging to the same class: smarter the most probable value: inference-based such as Bayesian formula or decision tree

21. April 13, 2015 21 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

22. April 13, 2015 22 How to Handle Noisy Data? Binning first sort data and partition into (equal-frequency) bins then one can smooth by bin means, smooth by bin median, smooth by bin boundaries , etc. Regression smooth by fitting the data into regression functions Clustering detect and remove outliers Combined computer and human inspection detect suspicious values and check by human (e.g., deal with possible outliers)

23. April 13, 2015 23 Simple Discretization Methods: Binning Equal-width (distance) partitioning 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 Divides the range into N intervals, each containing approximately same number of samples Good data scaling Managing categorical attributes can be tricky

24. April 13, 2015 24 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 equal-frequency (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

25. April 13, 2015 25 Regression x y y = x + 1 X1 Y1 Y1

26. April 13, 2015 26 Cluster Analysis

27. April 13, 2015 27 Data Preprocessing Why preprocess the data? Data cleaning Data integration and transformation Data reduction Discretization and concept hierarchy generation Summary

28. April 13, 2015 28 Data Integration Data integration: Combines data from multiple sources into a coherent store Schema integration: e.g., A.cust-id B. cust-# Integrate metadata from different sources Entity identification problem : Identify real world entities from multiple data sources, e.g., Bill Clinton = William Clinton 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

29. April 13, 2015 29 Handling Redundancy in Data Integration Redundant data occur often when integration of multiple databases Object identification : The same attribute or object may have different names in different databases Derivable data: One attribute may be a derived attribute in another table, e.g., annual revenue Redundant attributes may be able to be detected by correlation analysis Careful integration of the data from multiple sources may help reduce/avoid redundancies and inconsistencies and improve mining speed and quality

30. April 13, 2015 30 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 Attribute/feature construction New attributes constructed from the given ones

31. April 13, 2015 31 Data Transformation: Normalization Min-max normalization: to [new_min A , new_max A ] Ex. Let income range $12,000 to $98,000 normalized to [0.0, 1.0]. Then $73,000 is mapped to Z-score normalization ( : mean, : standard deviation): Ex. Let = 54,000, = 16,000. Then Normalization by decimal scaling Where j is the smallest integer such that Max(| |) < 1

32. April 13, 2015 32 Data Preprocessing Why preprocess the data? Data cleaning Data integration and transformation Data reduction Discretization and concept hierarchy generation Summary

33. April 13, 2015 33 Data Reduction Strategies Why data reduction? A database/data 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 Obtain a reduced representation of the data set that is much smaller in volume but yet produce the same (or almost the same) analytical results Data reduction strategies Data cube aggregation: Dimensionality reduction e.g., remove unimportant attributes Data Compression Numerosity reduction e.g., fit data into models Discretization and concept hierarchy generation

34. April 13, 2015 34 Data Cube Aggregation The lowest level of a data cube (base cuboid) 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 Queries regarding aggregated information should be answered using data cube, when possible

35. April 13, 2015 35 Attribute Subset Selection 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 Heuristic methods (due to exponential # of choices): Step-wise forward selection Step-wise backward elimination Combining forward selection and backward elimination Decision-tree induction

36. April 13, 2015 36 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}

37. April 13, 2015 37 Heuristic Feature Selection Methods There are 2 d possible sub-features of d features Several heuristic feature selection methods: Best single features under the feature independence assumption: choose by significance tests Best step-wise feature selection: The best single-feature is picked first Then next best feature condition to the first, ... Step-wise feature elimination: Repeatedly eliminate the worst feature Best combined feature selection and elimination Optimal branch and bound: Use feature elimination and backtracking

38. April 13, 2015 38 Given N data vectors from n -dimensions, find k n orthogonal vectors ( principal components ) that can be best used to represent data Steps Normalize input data: Each attribute falls within the same range Compute k orthonormal (unit) vectors, i.e., principal components Each input data (vector) is a linear combination of the k principal component vectors The principal components are sorted in order of decreasing significance or strength Since the components are sorted, the size of the data can be reduced by eliminating the weak components, i.e., those with low variance. (i.e., using the strongest principal components, it is possible to reconstruct a good approximation of the original data Works for numeric data only Used when the number of dimensions is large Dimensionality Reduction: Principal Component Analysis (PCA)

39. April 13, 2015 39 X1 X2 Y1 Y2 Principal Component Analysis

40. April 13, 2015 40 Data Preprocessing Why preprocess the data? Data cleaning Data integration and transformation Data reduction Discretization and concept hierarchy generation Summary

41. April 13, 2015 41 Summary Data preparation or preprocessing is a big issue for both data warehousing and data mining Discriptive data summarization is need for quality data preprocessing Data preparation includes Data cleaning and data integration Data reduction and feature selection Discretization A lot a methods have been developed but data preprocessing still an active area of research