WEKA classification likelihood of the classes

Question

I would like to know if there is a way in WEKA to output a number of \'best-guesses\' for a classification.

My scenario is: I classify the data with cross-validatio

Answer 1

I don't know if you can do it natively, but you can just get the probabilities for each class, sorted them and take the first three.

The function you want is distributionForInstance(Instance instance) which returns a double[] giving the probability for each class.

Answer 2

Weka's API has a method called Classifier.distributionForInstance() tha can be used to get the classification prediction distribution. You can then sort the distribution by decreasing probability to get your top-N predictions.

Below is a function that prints out: (1) the test instance's ground truth label; (2) the predicted label from classifyInstance(); and (3) the prediction distribution from distributionForInstance(). I have used this with J48, but it should work with other classifiers.

The inputs parameters are the serialized model file (which you can create during the model training phase and applying the -d option) and the test file in ARFF format.

public void test(String modelFileSerialized, String testFileARFF) 
    throws Exception
{
    // Deserialize the classifier.
    Classifier classifier = 
        (Classifier) weka.core.SerializationHelper.read(
            modelFileSerialized);

    // Load the test instances.
    Instances testInstances = DataSource.read(testFileARFF);

    // Mark the last attribute in each instance as the true class.
    testInstances.setClassIndex(testInstances.numAttributes()-1);

    int numTestInstances = testInstances.numInstances();
    System.out.printf("There are %d test instances\n", numTestInstances);

    // Loop over each test instance.
    for (int i = 0; i < numTestInstances; i++)
    {
        // Get the true class label from the instance's own classIndex.
        String trueClassLabel = 
            testInstances.instance(i).toString(testInstances.classIndex());

        // Make the prediction here.
        double predictionIndex = 
            classifier.classifyInstance(testInstances.instance(i)); 

        // Get the predicted class label from the predictionIndex.
        String predictedClassLabel =
            testInstances.classAttribute().value((int) predictionIndex);

        // Get the prediction probability distribution.
        double[] predictionDistribution = 
            classifier.distributionForInstance(testInstances.instance(i)); 

        // Print out the true label, predicted label, and the distribution.
        System.out.printf("%5d: true=%-10s, predicted=%-10s, distribution=", 
                          i, trueClassLabel, predictedClassLabel); 

        // Loop over all the prediction labels in the distribution.
        for (int predictionDistributionIndex = 0; 
             predictionDistributionIndex < predictionDistribution.length; 
             predictionDistributionIndex++)
        {
            // Get this distribution index's class label.
            String predictionDistributionIndexAsClassLabel = 
                testInstances.classAttribute().value(
                    predictionDistributionIndex);

            // Get the probability.
            double predictionProbability = 
                predictionDistribution[predictionDistributionIndex];

            System.out.printf("[%10s : %6.3f]", 
                              predictionDistributionIndexAsClassLabel, 
                              predictionProbability );
        }

        o.printf("\n");
    }
}

Answer 3

when you calculate a probability for the instance, how exactly do you do this?

I have posted my PART rules and data for the new instance here but as far as calculation manually I am not so sure how to do this! Thanks

EDIT: now calculated:

private float[] getProbDist(String split){

// takes in something such as (52/2) meaning 52 instances correctly classified and 2 incorrectly classified.

    if(prob_dis.length > 2)
        return null;

    if(prob_dis.length == 1){
        String temp = prob_dis[0];
        prob_dis = new String[2];
        prob_dis[0] = "1";
        prob_dis[1] = temp; 
    }

    float p1 = new Float(prob_dis[0]);
    float p2 = new  Float(prob_dis[1]);
    // assumes two tags
    float[] tag_prob = new float[2];

    tag_prob[1] = 1 - tag_prob[1];
    tag_prob[0] = (float)p2/p1;

// returns double[] as being the probabilities

return tag_prob;    
}

Answer 4

Not in general. The information you want is not available with all classifiers -- in most cases (for example for decision trees), the decision is clear (albeit potentially incorrect) without a confidence value. Your task requires classifiers that can handle uncertainty (such as the naive Bayes classifier).

Technically the easiest thing to do is probably to train the model and then classify an individual instance, for which Weka should give you the desired output. In general you can of course also do it for sets of instances, but I don't think that Weka provides this out of the box. You would probably have to customise the code or use it through an API (for example in R).