Keeley, Michael C., and Philip K. Robins. 1980. "Experimental Design, the Conlisk-Watts Assignment Model, and the Proper Estimation of Behavioral Response." Journal of Human Resources 15(4):480-498.

In the Seattle and Denver Income Maintenance Experiments (SIME/DIME), a variant of the Conlisk-Watts Assignment Model was used to allocate families to experimental treatments. This paper discusses the theory of the Conlisk-Watts Assignment Model, its application to SIME/DIME, and the implications of the model for estimating experimental effects. Unlike simple random assignment, the Conlisk-Watts model results in an experimental sample in which household characteristics are not orthogonal to experimental treatments and experimental treatments are not orthogonal to each other. Because of this nonorthogonality, the assignment variables must be included as explanatory variables in regression models that estimate experimental-control differences in order to avoid biased estimates of these differences. In addition, if the assignment variables affect response, they must be interacted with treatment variables in order to generalize the experimental findings to the national population and to obtain unbiased estimates of differential response among programs.

The authors are economists with SRI International. The research reported in this paper was performed pursuant to contracts with the States of Washington and Colorado, prime contractors for the Department of Health, Education and Welfare under contract numbers HEW-100-78-0005 and HEW-100-78-0004, respectively. The opinions expressed in this paper are those of the authors and should not be construed as representing the opinions or policies of the States of Washington or Colorado or any agency of the United States government. This paper is in part based on a more comprehensive paper entitled "The Design of Social Experiments: A Critique of the Conlisk-Watts Assignment Model and Its Application to the Seattle and Denver Income Maintenance Experiments." John Conlisk, Katherine Dickinson, Arthur Goldberger, Stanley Masters, Carl Morris, Robert Spiegelman, Nancy Tuma, Arlene Waksberg, Harold Watts, and Richard West commented on earlier versions of this paper, but do not necessarily share our conclusions.

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