Libraries provide implementation for the concepts of a particular domain. When programmers use a library, they do not work any more with the real-world concepts but with their implementation available within the used library. From this point of view, libraries represent the "reality" at which the programmers have to adapt. Ideally, in order to be easy to use, the implementation of concepts in libraries should accurately mirror the concepts and their relations from the real world. Unfortunately, this is not always realised and this results in a bias between the real-world concepts and their implementation. Depending on the kind of the bias, the users can adapt themselves or not to the library?s "reality". In this paper we propose a method to describe and evaluate the bias of the library implementation of real-world concepts expressed within an ontology. We use our method to describe several primitive bias classes in a formal framework and to discuss how can they affect the library?s users. We present our results with the help of bias examples which we (semi-)automatically identified in the Java standard library.