What is Data?

1. What is Data? Attributes • Collection of data objects and their attributes • An attribute is a property or characteristic of an object • Examples: eye color of a person, temperature, etc. • Attribute is also known as variable, field, characteristic, or feature • A collection of attributes describe an object • Object is also known as record, point, case, sample, entity, or instance Objects Data Mining: Concepts and Techniques

2. Transaction Data Market-Basket transactions Data Mining: Concepts and Techniques

3. Data Matrix • [ 1 4 5 6 • 2 3 3 4 • 2 2 1 1 ] Data Mining: Concepts and Techniques

4. % 1. Title: Iris Plants Database % • % 2. Sources: • % (a) Creator: R.A. Fisher • % (b) Donor: Michael Marshall (MARSHALL%PLU@io.arc.nasa.gov) • % (c) Date: July, 1988 • % @RELATION iris • @ATTRIBUTE sepallength NUMERIC • @ATTRIBUTE sepalwidth NUMERIC • @ATTRIBUTE petallength NUMERIC • @ATTRIBUTE petalwidth NUMERIC • @ATTRIBUTE class {Iris-setosa,Iris-versicolor,Iris-virginica} • The Data of the ARFF file looks like the following: @DATA 5.1,3.5,1.4,0.2,Iris-setosa 4.9,3.0,1.4,0.2,Iris-setosa 4.7,3.2,1.3,0.2,Iris-setosa Data Mining: Concepts and Techniques

5. uci machine learning repository • http://mlearn.ics.uci.edu/databases/ Data Mining: Concepts and Techniques

6. uci machine learning repository • http://archive.ics.uci.edu/ml/datasets.html/ Data Mining: Concepts and Techniques

7. Attribute Values • Attribute values are numbers or symbols assigned to an attribute • Distinction between attributes and attribute values • Same attribute can be mapped to different attribute values • Example: height can be measured in feet or meters • Different attributes can be mapped to the same set of values • Example: Attribute values for ID and age are integers • But properties of attribute values can be different • ID has no limit but age has a maximum and minimum value Data Mining: Concepts and Techniques

8. Discrete and Continuous Attributes • Discrete Attribute (Categorical Attribute) • Has only a finite or countably infinite set of values • Examples: zip codes, counts, or the set of words in a collection of documents • Often represented as integer variables. • Note: binary attributes are a special case of discrete attributes • Continuous Attribute (Numerical Attribute) • Has real numbers as attribute values • Examples: temperature, height, or weight. • Practically, real values can only be measured and represented using a finite number of digits. • Continuous attributes are typically represented as floating-point variables. Data Mining: Concepts and Techniques

9. Discrete and Continuous Attributes Data Mining: Concepts and Techniques

10. Chapter 3: Data Preprocessing • Why preprocess the data? • Data cleaning • Data integration and transformation • Data reduction • Discretization and concept hierarchy generation • Summary Data Mining: Concepts and Techniques

11. 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 Data Mining: Concepts and Techniques

12. 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. Data Mining: Concepts and Techniques

13. 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 Data Mining: Concepts and Techniques

14. Chapter 3: Data Preprocessing • Why preprocess the data? • Data cleaning • Data integration and transformation • Data reduction • Discretization and concept hierarchy generation • Summary Data Mining: Concepts and Techniques

15. Data Cleaning • Data cleaning tasks • Fill in missing values • Identify outliers and smooth out noisy data • Correct inconsistent data Data Mining: Concepts and Techniques

16. 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. Data Mining: Concepts and Techniques

17. Imputing Values to Missing Data • In federated data, between 30%-70% of the data points will have at least one missing attribute - data wastage if we ignore all records with a missing value • Remaining data is seriously biased • Lack of confidence in results • Understanding pattern of missing data unearths data integrity issues Data Mining: Concepts and Techniques

18. 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 attribute mean for all samples belonging to the same class to fill in the missing value: smarter • Use the most probable value to fill in the missing value: inference-based such as Bayesian formula or decision tree Data Mining: Concepts and Techniques

19. Noise • Noise refers to modification of original values • Examples: distortion of a person’s voice when talking on a poor phone and “snow” on television screen Two Sine Waves Two Sine Waves + Noise Data Mining: Concepts and Techniques

20. Outliers • Outliers are data objects with characteristics that are considerably different than most of the other data objects in the data set Data Mining: Concepts and Techniques

21. Outliers • Outliers are data objects with characteristics that are considerably different than most of the other data objects in the data set Data Mining: Concepts and Techniques

22. 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 Data Mining: Concepts and Techniques

23. How to Handle Noisy Data? • Binning method: • first sort data and partition into (equi-depth) bins • then one can 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 Data Mining: Concepts and Techniques

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 (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 Data Mining: Concepts and Techniques

25. 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. Data Mining: Concepts and Techniques

26. Cluster Analysis Data Mining: Concepts and Techniques

27. Regression y Y1 y = x + 1 Y1’ x X1 Data Mining: Concepts and Techniques

28. Chapter 3: Data Preprocessing • Why preprocess the data? • Data cleaning • Data integration and transformation • Data reduction • Discretization and concept hierarchy generation • Summary Data Mining: Concepts and Techniques

29. 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 Data Mining: Concepts and Techniques

30. Handling Redundant Data in Data Integration • Redundant data occur often when integration of multiple databases • The same attribute may have different names in different databases • One attribute may be a “derived” attribute in another table, e.g., annual revenue • Redundant data may be able to be detected by correlational analysis • Careful integration of the data from multiple sources may help reduce/avoid redundancies and inconsistencies and improve mining speed and quality Data Mining: Concepts and Techniques

31. 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 Data Mining: Concepts and Techniques

32. Data Transformation: Normalization • min-max normalization • z-score normalization • normalization by decimal scaling Where j is the smallest integer such that Max(| |)<1 Data Mining: Concepts and Techniques

33. Chapter 3: Data Preprocessing • Why preprocess the data? • Data cleaning • Data integration and transformation • Data reduction • Discretization and concept hierarchy generation • Summary Data Mining: Concepts and Techniques

34. Aggregation • Combining two or more attributes (or objects) into a single attribute (or object) • Purpose • Data reduction • Reduce the number of attributes or objects • Change of scale • Cities aggregated into regions, states, countries, etc • More “stable” data • Aggregated data tends to have less variability Data Mining: Concepts and Techniques

35. Aggregation Variation of Precipitation in Australia Standard Deviation of Average Monthly Precipitation Standard Deviation of Average Yearly Precipitation Data Mining: Concepts and Techniques

36. Sampling • Sampling is the main technique employed for data selection. • It is often used for both the preliminary investigation of the data and the final data analysis. • Statisticians sample because obtaining the entire set of data of interest is too expensive or time consuming. • Sampling is used in data mining because processing the entire set of data of interest is too expensive or time consuming. Data Mining: Concepts and Techniques

37. Sampling … • The key principle for effective sampling is the following: • using a sample will work almost as well as using the entire data sets, if the sample is representative • A sample is representative if it has approximately the same property (of interest) as the original set of data Data Mining: Concepts and Techniques

38. Types of Sampling • Simple Random Sampling • There is an equal probability of selecting any particular item • Sampling without replacement • As each item is selected, it is removed from the population • Sampling with replacement • Objects are not removed from the population as they are selected for the sample. • In sampling with replacement, the same object can be picked up more than once • Stratified sampling • Split the data into several partitions; then draw random samples from each partition Data Mining: Concepts and Techniques

39. Sample Size 8000 points 2000 Points 500 Points Data Mining: Concepts and Techniques

40. Sampling Cluster/Stratified Sample Raw Data Data Mining: Concepts and Techniques

41. Curse of Dimensionality • When dimensionality increases, data becomes increasingly sparse in the space that it occupies • Definitions of density and distance between points, which is critical for clustering and outlier detection, become less meaningful • Randomly generate 500 points • Compute difference between max and min distance between any pair of points Data Mining: Concepts and Techniques

42. Dimensionality Reduction • Purpose: • Avoid curse of dimensionality • Reduce amount of time and memory required by data mining algorithms • Allow data to be more easily visualized • May help to eliminate irrelevant features or reduce noise • Techniques • Principle Component Analysis • Singular Value Decomposition • Others: supervised and non-linear techniques Data Mining: Concepts and Techniques

43. Feature Subset Selection • Another way to reduce dimensionality of data • Redundant features • duplicate much or all of the information contained in one or more other attributes • Example: purchase price of a product and the amount of sales tax paid • Irrelevant features • contain no information that is useful for the data mining task at hand • Example: students' ID is often irrelevant to the task of predicting students' GPA Data Mining: Concepts and Techniques

44. Feature Creation • Create new attributes that can capture the important information in a data set much more efficiently than the original attributes • Three general methodologies: • Feature Extraction • domain-specific • Mapping Data to New Space • Feature Construction • combining features Data Mining: Concepts and Techniques

45. 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 Data Mining: Concepts and Techniques

46. Chapter 3: Data Preprocessing • Why preprocess the data? • Data cleaning • Data integration and transformation • Data reduction • Discretization and concept hierarchy generation • Summary Data Mining: Concepts and Techniques

47. 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 Data Mining: Concepts and Techniques

48. 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). Data Mining: Concepts and Techniques

49. Concept hierarchy generation for categorical data • Specification of a partial ordering of attributes explicitly at the schema level by users or experts • Specification of a portion of a hierarchy by explicit data grouping • Specification of a set of attributes, but not of their partial ordering • Specification of only a partial set of attributes Data Mining: Concepts and Techniques

50. Specification of a set of attributes Concept hierarchy can be automatically generated based on the number of distinct values per attribute in the given attribute set. The attribute with the most distinct values is placed at the lowest level of the hierarchy. 15 distinct values country 65 distinct values province_or_ state 3567 distinct values city 674,339 distinct values street Data Mining: Concepts and Techniques