The present invention may include performing a first measurement on a wafer of a first lot of wafers via an omniscient sampling process, calculating a first set of process tool correctables utilizing one or more results of the measurement performed via an omniscient sampling process, randomly selecting a set of field sampling locations of the wafer of a first lot of wafers, calculating a second set of process tool correctables by applying an interpolation process to the randomly selected set of field sampling locations, wherein the interpolation process utilizes values from the first set of process tool correctables for the randomly selected set of field sampling locations in order to calculate correctables for fields of the wafer of the first lot not included in the set of randomly selected fields, and determining a sub-sampling scheme by comparing the first set of process tool correctables to the second set of correctables.