1. A computer-readable-medium containing computer-executable instructions for performing a method for assuring the quality of a semiconductor processing tool, the semiconductor processing tool coupled to a process tool controller, the method comprising: (a) measuring a first set of values for a first set of one or more performance variables, the one or more performance variables being adapted to measure the performance of the semiconductor processing tool; (b) storing the first set of values in a first computer-readable medium coupled to the process tool controller; (c) measuring a second set of values for the first set of one or more performance variables; (d) storing the second set of values in a second computer-readable medium coupled to the process tool controller; and (e) retrieving the first set of values from the first computer-readable medium after performing element (d).