title: Mahout - Item exists in test data but not training data
body: <p>I am trying to evaluate a simple item-based recommender using <code>PearsonCorrelationSimilarity</code>. 
  I load the <code>DataModel</code> from a file that contains userid, itemid, preference, timestamp (in this order)
  My code looks something like that:</p>
  
  <pre><code>DataModel model = new FileDataModel(new File("FILE_NAME"));
  RecommenderEvaluator evaluator = new AverageAbsoluteDifferenceRecommenderEvaluator();
  
  RecommenderBuilder recommenderBuilder = new RecommenderBuilder() {
                      @Override
                      public Recommender buildRecommender(DataModel model) throws TasteException {
                          ItemSimilarity similarity = new PearsonCorrelationSimilarity(model);
                          Optimizer optimizer = new ConjugateGradientOptimizer();
                          return new KnnItemBasedRecommender(model, similarity, optimizer, N);
                      }
  
                  };
  score = evaluator.evaluate(recommenderBuilder, null, model, 0.7, 1.0);
  </code></pre>
  
  <p>When I run it I am getting lot's of </p>
  
  <pre><code>INFO eval.AbstractDifferenceRecommenderEvaluator: Item exists in test data but not training data:
  </code></pre>
  
  <p>Does this have to do something with my <code>DataModel</code> or with the evaluator. I've tried with both <code>RMSRecommenderEvaluator</code> and <code>AverageAbsoluteDifferenceRecommenderEvaluator</code> but I am getting the same INFO notice. I also tried using <code>RandomUtils.useTestSeed();.</code> 
  When I run the same using <code>UserSimilarity</code> metrics, I don't have this issue. </p>
  
  <p>My question is will this affect my evaluation results? </p>
  
  <p>Thank you.
  Dragan</p>