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Hello everyone, I will explain Cpk. For your convenience, both Chinese and English versions are provided.
大家好，我为大家带来Cpk的解释。为了阅读方便，这里提供了中文和英文版本。
English (USA)：
Why a Cpk Greater Than 1.33 Is Considered Acceptable
Cpk (Process Capability Index) is a statistical measure used to determine how well a process performs relative to its specification limits. A Cpk value of 1.33 or higher is widely regarded as acceptable in most industries because it strikes a good balance between process capability and defect rate.
Reasons Behind the 1.33 Threshold:
Six Sigma Standards:
While a Cpk of 2.0 (equivalent to a Six Sigma level) is ideal for critical processes, a Cpk of 1.33 represents a practical and achievable standard that balances cost and quality for non-critical processes.
Statistical Confidence:
A Cpk of 1.33 corresponds to a process where 99.99% of products fall within quality specification limits. This level ensures high customer satisfaction while minimizing rework and scrap.
Industry Standards:
In regulated industries such as pharmaceuticals, a minimum Cpk of 1.33 is often required by regulatory guidelines and customer agreements.
Process Variability:
A Cpk below 1.33 suggests that the process may be operating too close to its specification limits, making it more susceptible to shifts and variations that could lead to defects.
Safety Margin:
A Cpk of 1.33 ensures that the process not only meets quality standards but also incorporates a safety margin to account for variability and prevent defects. Specifically, a Cpk of 1.33 implies that the process output falls within ±4σ (sigma) from the mean, with a defect rate of approximately 63 parts per million (ppm).
In summary, a Cpk of 1.33 serves as the threshold for a "capable and reliable process" in most industries, ensuring efficiency and quality without incurring excessive costs.
Cpk Value, Yield, Defect Rate (PPM), and Sigma Level Comparison Table
Cpk Value
Sigma Level
Yield (%)
Defect Rate (PPM)
0.50
1.5
86.64%
133,614
0.67
2.0
95.45%
45,500
1.00
3.0
99.73%
2,700
1.33
4.0
99.9937%
63
1.67
5.0
99.99994%
0.57
2.00
6.0
99.9999998%
0.002
This table illustrates how higher Cpk values correspond to higher sigma levels, higher yields, and significantly lower defect rates.
中文版：
为什么Cpk大于1.33被视为可接受？
Cpk（过程能力指数）是一种统计指标，用于衡量过程在规格限范围内的执行能力。在大多数行业中，Cpk值达到1.33或更高被认为是可接受的，因为它在过程能力与缺陷率之间实现了较好的平衡。
1.33标准的主要原因：
六西格玛标准：
虽然Cpk达到2.0（六西格玛水平）是关键过程的理想目标，但Cpk为1.33代表了非关键过程中成本与质量平衡的实用且可实现的标准。
统计置信度：
Cpk为1.33对应99.99%的产品落在质量规格限内的过程能力。这一水平确保了较高的客户满意度，同时最大程度减少了返工和报废。
行业标准：
在制药等受监管的行业，法规和客户要求通常将Cpk值1.33作为可接受过程能力的最低标准。
过程变异性：
Cpk低于1.33表示过程运行过于接近规格限，更容易受到波动和偏移的影响，从而导致缺陷。
安全边际：
Cpk为1.33不仅意味着过程符合质量标准，还提供了应对变异的安全边际，避免缺陷产生。具体而言，Cpk为1.33表示过程输出在均值±4σ（西格玛）范围内，缺陷率约为63 ppm。
总而言之，Cpk值1.33是大多数行业中“过程稳定可靠”的门槛，能够在保证效率和质量的同时避免过高成本。
Cpk值、合格率、缺陷率（PPM）与西格玛水平对照表
Cpk值
西格玛水平
合格率
缺陷率（PPM）
0.50
1.5
86.64%
133,614
0.67
2.0
95.45%
45,500
1.00
3.0
99.73%
2,700
1.33
4.0
99.9937%
63
1.67
5.0
99.99994%
0.57
2.00
6.0
99.9999998%
0.002
上表展示了Cpk值越高，对应的西格玛水平越高，合格率越高，缺陷率显著降低。