title: Missing values for nominal attribute in Weka
body: <p>I have a data set and I am doing classification using <a href="http://www.cs.waikato.ac.nz/ml/weka/" rel="nofollow noreferrer">Weka</a> NaiveBayes classifier. I have 14 attributes, some of which are nominals.</p>
  
  <p>In only one of these attributes, I have some missing values. What I have done so far is that I have left them as missing values, and I know that Weka replaces those values automatically (a question is asked <a href="https://stackoverflow.com/questions/18230939/remove-missing-values-in-weka">here</a> about that ).</p>
  
  <p>I mean, the values for this attribute are empty in my feature file, and when I create the ARFF file, I see "?" between the two commas.</p>
  
  <p>Now, I have two possibilities: 
  1) Let them be filled by Weka automatically.
  2) Replace them by "NULL".</p>
  
  <p>The problem is that in the first case, the classifier works better. Now, I am wondering if it is allowed to let them be replaced by Weka? Or should I use the second approach, even though I get worse results?</p>
  
  <p>I mean, "when" should we let Weka replace the missing values? and when not?</p>
  
  <p>Meanwhile, the feature which has missing values represents the WordNet <a href="http://wordnet.princeton.edu/man/lexnames.5WN.html" rel="nofollow noreferrer">supersense</a> of the words and when it is empty, it means that the instance is, for example, a preposition, or a WH question.</p>
  
  <p>Thanks in advance,</p>