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Establishing Acceptance Limits for Probability of Passing Multiple Stage Tests in Process Validation through a Process Capability Approach
Author: Pramote Cholayudth

INTRODUCTION
Process capability index (CpK) is probably the most recognized traditional process performance index used in pharmaceutical technology. It is widely employed in quality by design (QbD), process validation, and annual product review activities (1, Figure 1). In process validation, the CpK (Figure 2) for a certain critical quality attribute (CQA) is computed to measure the process’ degree of data consistency in comparison with specification limits. One of the disadvantages to this dimensionless value is its limitation to communicate the exact meaning. For example, a common “rule of thumb” is that the process is poor if CpK < 1.0, good if CpK > 1.33 or excellent if CpK > 2.0. This kind of rule lacks a direct probabilistic interpretation. The solution to such limitation may be answered by the defect rate concept...

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