Download Chapter 7: Transformations

Chapter 7: Transformations
Attribute Selection
• Adding irrelevant attributes confuses learning algorithms---so avoid
such attributes
• Both divide-and-conquer and separate-and-conquer algorithms
suffer from this; Naïve Bayes does not suffer
• So first choose the attributes to be considered and then proceed--dimensionality reduction
• Scheme independent selection:
– Just enough attributes to divide up the instance space in a way that
separates all the training instances: For example, in Table 1, if we were
to drop outlook, instance 1 and 4 will be inseparable-not good. --- very
tedious procedure
• Using machine learning algorithms for attribute selection
– Decision tree: Apply DT on all attributes, and select only that are
actually used in the decisions---the selected attributes can then be used
in another chosen learning algorithm
– Use linear SVM algorithm that ranks attributes based on weights to
choose the attributes---recursive feature elimination
– Using instance-based learning methods
• Sample instances randomly from the training set
• Check neighboring records of the same and different classes (near hits and
near misses)
• If a near hit has a different value for a certain attribute, that attribute appears
to be irrelevant---reduce its weight
• If a near miss, has a different value, the attribute appears to be relevant and
its weight should be increased
• After repeating this procedure many times, selection takes place---only
attributes with +ve weights are chosen.
• Searching the attribute space:
–
–
–
–
Fig 7.1
Forward selection (start with empty set and keep expanding)
Backward elimination (start with all, and start eliminating one by one)
Bidirectional search---combination of the above two
• Scheme-specific selection
– Cross-validation is used to measure the effectiveness of a subset of attributes
Discretizing Numeric Attributes
•
•
•
•
•
•
Global discretization: Used in 1R learning scheme: Sort the instances by the
attribute’s value and assign the value into ranges at the points that class
value changes---keeping some minimum instance coverage criteria
Local discretization: Used in decision trees: When a specific attribute is
used to split a node, a decision is made on the value at which this break
could take place
Transforming numeric attribute into k binary variables
Unsupervised discretization: Not taking the classes of the training set--break the value range into some intervals---e,g., equal-interval binning or
equal-frequency binning---runs the risk of destroying distinctions within an
interval or bin
Supervised discretization---takes classes into account while making
intervals
Proportional k-interval discretization: #of bins chosen as square root of #of
instances with equal-frequency binning is found to be excellent
Entropy-based Discretization
• One example: Order the values of the attribute, and for each
possible break-point determine the information gain (p. 298-299).
Split at the point where this value is the smallest.
– For all values, find the smallest (A);
– Repeat this procedure for each of the parts formed by the breaking at A;
– Repeat this step recursively until a stopping criteria is met
Some Useful Transformations
• Examples:
– Subtracting one date attribute from another to obtain a new age
attribute
– Converting two attributes A and B to A/B, a new attribute representing
the ratio
– Reduce several nominal attributes to one by concatenating their vales,
producing a single k1xk2 value attribute
• Principal component analysis: Use a special coordinate system that
depends on the given cloud of points as follows: place the first axis
in the direction of greatest variance of the points to maximize the
variance along that axis; the 2nd axis in perpendicular to it; in multidimensional case, choose the 2nd axis that maximizes variance
along that axis; and so on; finally, choose the ones that contribute to
the highest variance---the principal components
•
•
http://en.wikipedia.org/wiki/Principal_components_analysis
http://csnet.otago.ac.nz/cosc453/student_tutorials/principal_components.pdf
Random Projections
• Since PCA is expensive (cubic in the #of
dimensions), alternative is to a random
projection of the data into a subspace with
a predetermined number of dimensions
Text to attribute vector
• Convert a document to a vector of words
that occur in the document---it could be
the frequency of the words or just the
absence/presence of the word
• In other words, a document is
characterized by the words that appear
often in it.
Time series
• Some times, we may replace the attributes
by the difference in successive values, etc.
This is time series.
Automatic Data Cleansing
• Data mining techniques themselves can sometimes help to solve the
problem of cleansing the corrupted data
• By discarding misclassified instances from the training set,
relearning, and then repeating until there are no more misclassified
instances, decision trees induced from data can be improved
• Robust regression---by removing outliers, linear regression is
improve
Combining Multiple Models
• Bagging, boosting, and stacking are prominent methods to combine
multiple models
• Bagging: Models receive equal weight---output of each model is a
majority value, for example.
• Boosting: Similar to bagging except that it assigns different weights
to different model outputs
• Option tree (Fig. 7.10) and Fig. 7.11 (-ve means play=yes; + ve
means play=no;)