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Cpk Formula For One-Sided Specification:
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Cpk (Process Capability Index) for one-sided specification measures how well a process meets a single specification limit. It indicates the process capability when only one limit (upper or lower) is specified, showing how close the process mean is to the specification limit relative to process variation.
The calculator uses the Cpk formula for one-sided specification:
Where:
Explanation: The formula calculates how many standard deviations the specification limit is from the process mean, divided by 3 to represent the process capability index.
Details: Cpk calculation is crucial for quality control and process improvement. It helps determine if a process is capable of meeting specifications, identifies areas for improvement, and ensures consistent product quality. A higher Cpk value indicates better process capability.
Tips: Enter the specification limit, process mean, and standard deviation. All values must use consistent units. Standard deviation must be greater than zero for valid calculation.
Q1: What is a good Cpk value?
A: Generally, Cpk ≥ 1.33 indicates a capable process. Cpk ≥ 2.0 is considered excellent. Values below 1.0 may indicate process improvement is needed.
Q2: When should one-sided Cpk be used?
A: Use one-sided Cpk when only one specification limit is relevant (e.g., maximum impurity level, minimum strength requirement).
Q3: How is one-sided Cpk different from two-sided Cpk?
A: Two-sided Cpk considers both upper and lower specification limits and uses the minimum value, while one-sided Cpk focuses on a single specification limit.
Q4: What are the limitations of Cpk?
A: Cpk assumes normal distribution of data and may not be accurate for non-normal processes. It also requires stable process conditions.
Q5: Can Cpk be negative?
A: Yes, negative Cpk values indicate the process mean is beyond the specification limit, meaning the process is not capable of meeting requirements.