Fault-based testing strategies test software by focusing on specific, common types of errors. The coupling effect states that test data sets that detect simple types of faults are sensitive enough to detect more complex types of faults. This paper describes empirical investigations into the coupling effect over a specific domain of software faults. All the results from this investigation support the validity of the coupling effect. The major conclusion from this investigation is that by explicitly testing for simple faults, we are also implicitly testing for more complicated faults. This gives us confidence that fault-based testing is an effective means of testing software.