The effect of a planned process change on product performance is to be evaluated by implementing the change directly on the manufacturing floor and assessing the results. It is shown that, instead of making a one-time change and comparing the results after the change with those before, a statistically meaningful evaluation generally requires“back and forth”comparisons. The exact manner in which this is done will depend upon practical considerations. The concepts are illustrated by an example dealing with the manufacture of an electronic component, where the claim that a process change would lead to a reduction in cost without an appreciable decline in quality had to be verified. Sample size requirements, based upon an initial determination and a sequential decision plan, are also described.