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First provide a summary of the paper, and then address the following criteria: Quality, clarity, originality and significance. This paper provides some theoretical analysis on Positive and Unlabeled Data learning (PU), when only the positive instances and unlabeled instances are available. The authors show that learning from PU is equivalent to a cost-sensitive classification task if the label prior is known. The main contribution of the paper is that, the authors show that using any convex loss leads to inconsistent classifier for PU tasks. Instead, using a non-convex ramp loss gives a consistent estimator. This theoretical justification is supported by experiments, which demonstrate that adopting hinge loss of SVM may result in very bad classification error comparing to using the non-convex ramp loss.