  This paper analyzes the working or default assumptions researchers in the
formal, statistical, and case study traditions typically hold regarding the
sources of unexplained variance, the meaning of outliers, parameter values,
human motivation, functional forms, time, and external validity. We argue that
these working assumptions are often not essential to each method, and that
these assumptions can be relaxed in ways that allow multimethod work to
proceed. We then analyze the comparative advantages of different combinations
of formal, statistical, and case study methods for various theory-building and
theory-testing research objectives. We illustrate these advantages and offer
methodological advice on how to combine different methods, through analysis and
critique of prominent examples of multimethod research.
