Statistical studies must be conducted to analyze the variation present in the results of each type of inspection, measurement system and test equipment identified in the control plan. The analytical methods and acceptance criteria used must comply with those in the measurement systems analysis reference manuals. Other analytical methods and acceptance criteria may be used if approved by the customer. Records of customer acceptance of alternative methods should be retained along with the results of alternative analysis of measurement systems (see Section 9.1.1.1) NOTE Prioritization of MSA studies should focus on critical or special characteristics of the product and process.