Regulatory guidelines method transfer

In robustness tests, the selected range between the levels should represent the variability that can occur when transferring the method (4, 5,16,18,25). However, specifications to estimate such variability are not given in regulatory documents, such as the ICH guidelines. Often the extreme levels are chosen based on personal experience, knowledge, or intuition. Sometimes they are defined as nominal level + x%. However, this approach based on relative variation is not appropriate because the absolute variation then depends on the value of the nominal level (18). Another systematic approach defines the levels based on the precision or the uncertainty with which they can be set (5, 18). The uncertainty can be estimated for the nominal factor level (18, 26). If the uncertainty or absolute error on a measured pH value... 

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