Weyuker and Jeng have investigated the conditions that affect the performance of partition testing and have compared analytically the fault-detecting ability of partition testing and random testing. Chen and Yu have generalized some of Weyuker and Jeng's results. In this paper, we extend the analysis to subdomain testing in which subdomains may overlap. We derive several results for a special case and demonstrate a technique to extend some of our results to more general cases. We believe that this technique should be very useful in further investigating the behavior of subdomain testing.