1. A method comprising: providing a simulation model to determine a random yield for a layout design, wherein the layout design includes a memory array with one or more redundant rows or columns, wherein the simulation model comprises a failure model for the memory array; determining a probability of failure of the memory array of the layout design, wherein the probability of failure of the memory array comprises accumulating a probability of failure of a two-dimensional array of bits, a probability of failure of a column of replaceable row elements and a probability of failure of a row of replaceable column elements through a sum of products of probabilities of fail and probabilities of no fail; and calculating, by using a processor, the random yield for the layout design based at least in part upon the accumulated probabilities of failure.