Classification in weka fails, caused by case sensitiveness of nominal values?

Question

I made a classifier to classify search queries into one of the following classes: {Artist, Actor, Politician, Athlete, Facility, Geo, Definition, QA}. I have two csv files: one for training the classifier (contains 300 queries) and one for testing the classifier (currently contains about 200 queries). When I use the trainingset and testset for training/evaluating the classifier with weka knowledgeflow, most classes reach a pretty good accuracy. Setup of Weka knowledge flow training/testing situation:

Weka training/testing setup

After training I saved the MultiLayer Perceptron classifier from the knowledgeflow into classifier.model, which I used in java code to classify queries.

When I deserialize this model in java code and use it to classify all the queries of the testing set CSV-file (using the distributionForInstance()-method on the deserialized classifier) in the knowledgeflow it classifies all 'Geo' queries as 'Facility' queries and all 'QA' queries as 'Definition' queries. This surprised me a bit, as the ClassifierPerformanceEvaluator showed me a confusion matrix in which 'Geo' and 'QA' queries scored really well and the testing-queries are the same (the same CSV file was used). All other query classifications using the distributionForInstance()-method seem to work normally and so show the behavior that could be expected looking at the confusion matrix in the knowledgeflow. Does anyone know what could be possible causes for the classification difference between distributionForInstance()-method in the java code and the knowledgeflow evaluation results?

One thing that I can think of is the following: The testing-CSV-file contains among other attributes a lot of nominal value attributes in all-capital casing. When I print out the values of all attributes of the instances before classification in the java code these values seem to be converted to lower capital letters (it seems like the DataSource.getDataSet() method behaves like this). Could it be that the casing of these attributes is the cause that some instances of my testing-CSV-file get classified differently? I read in Weka specification that nominal value attributes are case sensitive. I change these values to uppercase in the java file though, as weka then throws an exception that these values are not predefined for the nominal attribute.

possible duplicate of Weka java library: how to get string representation of classified instance? — Has QUIT--Anony-Mousse
Although the other question has indeed the same author and does concern the same project, I do believe that both questions are actually different. — Niek Tax

kaz kaz · Accepted Answer · 2012-05-25T17:13:57

Weka is likely using the same class in the knowledge flow as in your weka code to interpret the csv. This is why it works (produces data sets -- Instances objects -- that match) without tweaking and fails when you change things: the items don't match any more. This is to say that weka is handling the case of the input strings consistently, and does not require you to change it.

Check that you are looking at the Error on Test Data value and not the Error on Training Data value in the knowledge flow output, because the second one will be artificially high given that you built the model using those exact examples. It is possible that your classifier is performing the same in both places, but you are looking at different statistics.

Classification in weka fails, caused by case sensitiveness of nominal values?

1 Answers