Finding out wrongly classified instances when using WEKA

Question

I am using GUI version of WEKA and I am classifying using the Naive Bayes. Can anyone please let me know how to find out which instances are misclassified.

Answer 1

1. Go to classify tab in Weka explorer
2. Click more options...
3. Check output predictions
4. Click OK

Hope that helps.

Answer 2

I faced this very same problem earlier and I tackle it just fine now. What I do, is the following:

1. Make one String attribute that assigns each instance a unique ID. I have assigned the names of the documents to each of my instances.
2. Generate the WEKA supported .arff file.
3. Whenever you have to run a classifier on this .arff data, you will notice that you have to exclude the Instance ID attribute. If you don't do this, Weka will pop-up an error saying that the classifier cannot process String attributes. Instead of excluding, run filter StringToNominal on the InstanceID.
4. Now, as said by @Rushdi, click "More Options" on the classify tab.
5. Check Output predictions on the "Classifier Evaluation Options" pop-up.
6. Enter the Attribute number of the Instance ID in the "Output additional attributes" box.
7. Run the classifier on the whole data, excluding the Instance ID attribute. (Most classifiers have this as an option called "StartSet" in "Ranker" for example which I use along with SMO classifier.)
8. If you've done everything properly so far, you will see all the instances listed along with their real and predicted output values and also the Instance ID which can tell you exactly which documents were incorrectly classified.

Hope this helps someone. Good Luck!