Bracht, G. H. and Glass, G. V.The external validity of experiments. American Educational Research Journal, 1968, 5, 437-474.
2.
Campbell, D. T. and Stanley, J. C.Experimental and quasi-experimental designs for research on teaching. In N. L. Gage (Ed). Handbook of research on teaching. Chicago: Rand McNally, 1963.
3.
Von Wright, G. H.Explanation and understanding . Ithaca, N. Y.: Cornell Univ. Press, 1971.