The problem of detection of defective factors in a large population consisting of defective and non-defective factors has been tackled in various ways. One such an approach is Two-type Step-wise Group Screening Designs with Errors in Observations.In the first step of this design we assume all factors have equal prior probability of being defective and obtain an expression for the expected number of runs required to carry out the whole experiment.The expression obtained is used to generate some tables using numerical approximation. In the second step of the design we assume the factors have a different prior probability of being defective and an expression for the expected number of runs required to analyze the whole experiment is obtained. The expression obtained is used to generate tables again using numerical method.The tables are used to evaluate the efficiency of the design.