1. A computer readable storage medium comprising a computer readable program for computing threshold voltage degradation of transistors in an array of memory cells, wherein the computer readable program when executed on a computer causes the computer to perform the steps of: selecting a process that models an expected usage of the array; running the process to populate the array with data over time to simulate the expected usage of the array; compiling data that detail different durations at which each of the memory cells in the array stores 1 or at which each of the memory cells in the array stores 0; and for each separate grouping of memory cells that share a common duration of the different compiled durations, determining a threshold voltage degradation for each transistor in the corresponding grouping of cells based on at least one biased temperature instability model.