1. Data Preprocessing Dr. Bernard Chen Ph.D. University of Central Arkansas

2. Outline Introduction Descriptive Data Summarization Data Cleaning Missing value Noise data Data Integration Redundancy Data Transformation

3. Knowledge Discovery (KDD) Process Data mining—core of knowledge discovery process Data Cleaning Data Integration Databases Data Warehouse Task-relevant Data Selection Data Mining Pattern Evaluation

4. Knowledge Process 1. Data cleaning – to remove noise and inconsistent data 2. Data integration – to combine multiple source 3. Data selection – to retrieve relevant data for analysis 4. Data transformation – to transform data into appropriate form for data mining 5. Data mining 6. Evaluation 7. Knowledge presentation

5. Why Preprocess the data Image that you are a manager at ALLElectronics and have been charger with analyzing the company’s data Then you realize: Several of the attributes for carious tuples have no recorded value Some information you want is not on recorded Some values are reported as incomplete, noisy, and inconsistent Welcome to real world!!

6. 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

7. 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

8. Why Is Data Dirty? Noisy data (incorrect values) may come from Faulty data collection instruments Human or computer error at data entry Errors in data transmission

9. Why Is Data Dirty? Inconsistent data may come from Different data sources Functional dependency violation (e.g., modify some linked data) Duplicate records also need data cleaning

10. 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 extraction, cleaning, and transformation comprises the majority of the work of building a data warehouse

11. 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

12. Forms of Data Preprocessing

13. Outline Introduction Descriptive Data Summarization Data Cleaning Missing value Noise data Data Integration Redundancy Data Transformation

14. Descriptive data summarization Motivation To better understand the data: central tendency, variation and spread Data dispersion characteristics median, max, min, quantiles, outliers, variance, etc.

15. Descriptive data summarization Numerical dimensions correspond to sorted intervals Data dispersion: analyzed with multiple granularities of precision Boxplot or quantile analysis on sorted intervals

16. Measuring the Central Tendency Mean Average value of the data Median Middle value of the data Mode Value that occurs most frequently in the data Dataset with one, two or three modes are respectively called unimodal, bimodal, and trimodal

17. Symmetric vs. Skewed Data

18. Measuring the Dispersion of Data Quartiles, outliers and boxplots The median is the 50 th percentile Quartiles: Q 1 (25 th percentile), Q 3 (75 th percentile) Inter-quartile range (IQR): IQR = Q 3 – Q 1 Outlier: usually, a value higher/lower than 1.5 x IQR

19. 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

20. Boxplot Analysis

21. The median for each dataset is indicated by the black center line, and the first and third quartiles are the edges of the red area, which is known as the inter-quartile range (IQR). Points at a greater distance from the median than 1.5 times the IQR are plotted individually as asterisks.

22. Histogram Analysis Graph displays of basic statistical class descriptions Frequency histograms A univariate graphical method Consists of a set of rectangles that reflect the counts or frequencies of the classes present in the given data

23. Histogram Analysis

24. Outline Introduction Descriptive Data Summarization Data Cleaning Missing value Data Integration Redundancy Data Transformation

25. Data Cleaning Importance “Data cleaning is one of the three biggest problems in data warehousing”—Ralph Kimball “Data cleaning is the number one problem in data warehousing”—DCI survey

26. Data Cleaning Data cleaning tasks Fill in missing values Identify outliers and smooth out noisy data

27. Missing 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 It is important to note that, a missing value may not always imply an error. (for example, Null-allow attri. )

28. 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

29. How to Handle Missing Data? 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

30. Outline Introduction Descriptive Data Summarization Data Cleaning Missing value Noise data Data Integration Redundancy Data Transformation

31. Data integration Data integration: Combines data from multiple sources into a coherent store

32. Data integration problems Schema integration: e.g., A.cust-id  B.cust-# Integrate metadata from different sources 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

33. Redundant data 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

34. Redundant data 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

35. Outline Introduction Descriptive Data Summarization Data Cleaning Missing value Noise data Data Integration Redundancy Data Transformation

36. Data Transformation can involve the following: Smoothing: remove noise from the data, including binning, regression and clustering Aggregation Generalization Normalization Attribute construction

37. Normalization

38. Min-max normalization Z-score normalization Decimal normalization

39. Min-max 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

40. Z-score normalization Z-score normalization (μ: mean, σ: standard deviation): Ex. Let μ = 54,000, σ = 16,000. Then