Unfortunately, PROC FREQ doesn’t provide RISKDIFF analysis for two-way tables when there is a zero-frequency column(both treatment arms have no positive responses). In the meantime the work-around to assign very small weights to the zero-frequency cells in your two-way table is the only option. When you use this work-around, the following confidence limit types are available for the risk difference: Agresti-Caffo, Hauck-Anderson, Miettinen-Nurminen (score), and Newcombe. Of course the traditional/Wald confidence limits are also available, but equal (0,0) in the case of a zero-frequency column where the estimate of the risk difference is 0.
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