Reference the quality literature on statistical process control, also known as “SPC.”  There are many excellent books on the subject, particularly the one by “Grant and Levenworth.”  Process improvement and optimization is often accomplished by way of statistically designed experiments, or “DOE” as it is known.  To this end, the work of “George Box” is recommended as well as that of “Douglas Montgomery.”  A good place to get a birds-eye view of many different quality methods is to look through Juran’s Handbook of Quality.