A method includes determining a sampling region in a sample space, generating samples in the sampling region without generating samples outside the sampling region, and simulating a performance of a device using the generated samples as input data. The sample space is defined by a plurality of variables associated with the device. Values of the plurality of variables in the sampling region having lower probabilities to meet a specification of the device than values of the plurality of variables outside the sampling region. The method is performed at least partially by at least one processor.