Data Preprocessing: Data Reduction Techniques Compiled By: Umair Yaqub Lecturer Govt. Murray College Sialkot

2 Data Reduction Method (1): Regression  Regression Linear regression: Data are modeled to fit a straight line Often uses the least-square method to fit the line Y =  +  X Two parameters,  and  specify the line and are to be estimated by using the data at hand. using the least squares criterion to the known values of Y 1, Y 2, …, X 1, X 2, …. Multiple regression: allows a response variable Y to be modeled as a linear function of multidimensional feature vector Y = b0 + b1 X1 + b2 X2

3 Data Reduction Method (2): Histograms  Divide data into buckets and store average (sum) for each bucket  Singleton Represent a single attribute/frequency pair  Continuous ranges may also be represented  Partitioning rules: Equal-width: equal bucket range Equal-frequency (or equal-depth)

4 Data Reduction Method (3): Clustering  Partition data set into clusters based on similarity, and store cluster representation (e.g., centroid and diameter) only  Can have hierarchical clustering and be stored in multi-dimensional index tree structures  There are many choices of clustering definitions and clustering algorithms

5 Data Reduction Method (3): Clustering..  consider the root of a B+-tree, with pointers to the data keys 986, 3396, 5411, 8392, and  Suppose that the tree contains 10,000 tuples with keys ranging from 1 to  The data in the tree can be approximated by an equal-frequency histogram of six buckets for the key ranges 1 to 985, 986 to 3395, 3396 to 5410, 5411 to 8391, 8392 to 9543, and 9544 to  Each bucket contains roughly 10,000/6 items.

6 Sampling  Allow a mining algorithm to run in complexity that is potentially sub-linear to the size of the data  Possible samples Simple random sample without replacement (size n)  All tuples are equally likely Simple random sample with replacement (size n) Cluster sample  Retrieving clusters samples of tuples Stratified samples  Approximate the percentage of each class (or subpopulation of interest) in the overall database  Used in conjunction with skewed data

7 Sampling…

8 Discretization and Concept Hierarchy  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).

9 Discretization and Concept Hierarchy Generation for Numeric Data  Typical methods: All the methods can be applied recursively Binning  Top-down split, unsupervised Histogram analysis  Top-down split, unsupervised Clustering analysis  Either top-down split or bottom-up merge, unsupervised

10 Concept Hierarchy Generation for Categorical Data  Specification of a partial/total ordering of attributes explicitly at the schema level by users or experts street < city < state < country  Specification of a hierarchy for a set of values by explicit data grouping {Urbana, Champaign, Chicago} < Illinois  Specification of only a partial set of attributes E.g., only street < city, not others  Automatic generation of hierarchies (or attribute levels) by the analysis of the number of distinct values E.g., for a set of attributes: {street, city, state, country}

11 Automatic Concept Hierarchy Generation  Some hierarchies can be automatically generated based on the analysis of the number of distinct values per attribute in the data set The attribute with the most distinct values is placed at the lowest level of the hierarchy Exceptions, e.g., weekday, month, quarter, year country province_or_ state city street 15 distinct values 365 distinct values 3567 distinct values 674,339 distinct values