We present a gauge study of an on-line metrology system for chemical-mechanical polishing and a 600 wafer run by run (RbR) control experiment enabled by on-line wafer measurement. The variability, reliability, and accuracy of the on-line metrology system are found to be very good. We show that a simple control approach provides a root-mean-squared error of less than 100 Angstrom. In contrast, using pilot wafers and sheet film equivalents to control a process results in a 39% decrease in performance, and that using fewer sites may increase variability and lead to an incorrect controlled thickness. We outline how an 8%-80% improvement in throughput, as well as several reductions in cost of ownership, are possible using on-line metrology in conjunction with run by run control.