I am doing semi-supervised classification with WEKA. The test set of my data (twitter) is unlabeled (no class assigned) but replaced with (?). I used WEKA to convert the test data from csv to arff file, which sets automatic datatype of 'string' to the class attribute. When I try to run it after creating the model with my train data it seems to give errors regarding the string datatype given to the unlabelled class attribute. My question is what datatype is suitable for the class attribute in '?' to avoid glitches?