classification in weka - ijskt.ijs.si/petrakralj/ips_dm_0910/handsonweka-part1.pdf ·...

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Classification in WEKA 2009/11/10

Petra Kralj Novak

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Practice with Weka

1. Build a decision tree with the ID3 algorithm on the lenses dataset, evaluate on a separate test set

2. Classification on the CAR dataset– Preparing the data– Building decision trees– Naive Bayes classifier– Understanding the Weka output

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Weka

Download

version

3.6

Weka is open source softwere for machine learning and data mining.

http://www.cs.waikato.ac.nz/ml/weka/

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Run Weka

Choose Explorer

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Exercise 1: Lenses dataset

• In the Weka data mining tool induce a decision

tree for the lenses dataset with the ID3

algorithm.

• Data:– lensesTrain.arff

– lensesTest.arff

• Compare the outcome with the manually

obtained results.

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Load the data

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Load the data - 2

lensesTrain.arff

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The data are loaded

Target variable

Choose

“Classify”

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Choose algoritem

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trees

Id3

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12

3

5

lensesTest.arff

4

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Decision tree

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Classification accuracy

Confusion

matrix

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Exercise 2: CAR dataset

• 1728 examples

• 6 attributes– 6 nominal

– 0 numeric

• Nominal target variable– 4 classes: unacc, acc, good, v-good

– Distribution of classes• unacc (70%), acc (22%), good (4%), v-good (4%)

• No missing values

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Preparing the data for WEKA - 1

Data in a spreadsheet

(e.g. MS Excel)

- Rows are examples

- Columns are attributes

- The last column is the target variable

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Preparing the data for WEKA - 2

Save as “.csv”- Careful with dots “.”,

commas “,” and semicolons “;”!

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Load the data

Target variable

Car.csv

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Choose algorithm J48

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Building and evaluating the tree

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Classified as

Actual values

Classification

accuracy

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Right

mouse

click

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Tree pruning

Set the minimal

number of objects

per leaf to 15

Parameters of the

algorithm

(right mouse click)

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Reduced

number of

leaves and

nodesEasier to interpret

Lower

classification

accuracy

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Naïve Bayes classifier

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