The process of determining the required test length for a desired level of confidence for pseudorandom testing using a random sampling without replacement model is examined. The differences between random and pseudorandom testing are discussed and developed. The strictly random testing model is shown to be inaccurate for high confidence testing of combinational circuits. A method of calculating the required test length for pseudorandom testing based upon fault detectabilities is described. The result provides a very accurate prediction of required test length applicable to self-test using pseudorandom inputs.

Algorithmic test generation for high fault coverage is an expensive and time-consuming process. As an alternative, circuits can be tested by applying pseudorandom patterns generated by a linear feedback shift register (LFSR). Although no fault simulation is needed, analysis of pseudorandom testing requires the circuit detectability profile.