Ahmann, J. S., Loomis, R. J., and Kelley, J. W.The evaluation of leadership training institutes. Fort Collins, Colorado: Colorado State University, 1971.
2.
Campbell, D. T. and Stanley, J. C.Experimental and quasi-experimental designs for research . Chicago: Rand-McNally, 1963.
3.
Webb, E. J., Campbell, D. T., Schwartz, R. D., and Sechrest, L.Unobtrusive measures: Nonreactive measures in the social sciences. Chicago : Rand-McNally, 1966.