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Freedman, D. A. (2006)  Statistical models: Theory and practice. Cambridge: Cambridge University Press: 2-13.
Freedman, D. A. (1997) From association to causation via regression. In V. R. McKim and S. P. Turner (Eds.) Causality in crisis?: Statistical methods and the search for causal knowledge in the social sciences. Notre Dame, IN: University of Notre Dame Press.
Pearl, J. (2000) Causality: Models, reasoning, and inference. Cambridge: Cambridge University Press.