  It is crucial for accurate model checking that the model be a complete and
faithful representation of the system. Unfortunately, this is not always
possible, mainly because of two reasons: (i) the model is still under
development and (ii) the correctness of implementation of some modules is not
established. In such circumstances, is it still possible to get correct answers
for some model checking queries?
  This paper is a step towards answering this question. We formulate this
problem for the Discrete Time Markov Chains (DTMC) modeling formalism and the
Probabilistic Computation Tree Logic (PCTL) query language. We then propose a
simple solution by modifying DTMC and PCTL to accommodate three valued logic.
The technique builds on existing model checking algorithms and tools, obviating
the need for new ones to account for three valued logic.
  One of the most useful and popular techniques for modeling complex systems is
through discrete event simulation. Discrete event simulators are essentially
code in some programming language. We show an application of our approach on a
piece of code that contains a module of unknown correctness.
  A preliminary version of this paper appears in the proceedings of Leveraging
Applications of Formal Methods, Verification and Validation: Foundational
Techniques (ISoLA 2016), LNCS 9952, Springer.
  Keywords: Probabilistic models, Probabilistic Model checking Three-valued
Logic, Discrete Time Markov Chain, Probabilistic Computation Tree Logic.
