Weka Experimenter 'Class attribute is not nominal' but data is processed from Explorer

Question

Good Evening,

I am working on a supervised classification task. I have a big arff file full of data in the format, "text", class. There are only two classes, E and I.

I can load this data into Weka Explorer, apply the StringToWordVector with TF-IDF on it, then using LibSVM classify it and get results. But I need to use 5x2 Cross-Validation and get the Area under the ROC Curve. So I save that processed data, open up Weka Experimenter, load it in, set it to 2 folds, 5 iterations, and then set the algorithm to libSVM.

When I go to the RUN tab and press start I get the following error:

18:31:18: Started

18:31:18: Class attribute is not nominal!

18:31:18: Interrupted

18:31:18: There was 1 error

I don't know why this is happening, what exactly the error is, or how to fix it. I google this error and it is not leading me to any solutions. I am not sure where I should go from here to fix this.

I can go back to Explorer, reload in that processed file, and classify it without any issues but I need to do it in Experimenter.

Answer 1

In my case, there were nominal attributes in the file. However, Weka expects these to be last, since they indicate the class that the record is being assigned to. Here's how I rearranged the data so that the nominal value was last:

1. In Explorer, open the arff file.
2. Click 'Edit...' then find the column which should be the class of each record.
3. Right click on the column header and select 'Attribute as class'.
4. Click 'Save...' and use this new dataset in Experimenter.

Works like a charm.