For an integrated circuit associated with a first plurality of parameters whose values are described by a first probability distribution function, a method for estimating a failure probability includes selecting a first plurality of samples, performing a first test to determine an outcome for each of the first plurality of samples and identifying failed samples. A second plurality of parameters is selected that has fewer parameters than the first plurality of parameters. The failed samples are clustered in the space of the second plurality of parameters using a computer-implemented cluster forming method that, in some cases, returns multiple clusters. The method also includes forming a probability distribution function for each of the clusters, forming a composite probability distribution function that includes a weighted combination of the first probability distribution function and the probability distribution function for each of the clusters. The method further includes selecting a second plurality of samples using the composite probability distribution function and performing a second test to determine an outcome for each of the second plurality of samples. A failure probability can then be computed.