今天分享的是質量英文，主題是-過程能力分析：Cp與Cpk攜手同行，作者是Cody Steele，中文為本公號創建人浦亮元翻譯，全文1538字，感謝閱讀與分享！
正文：
Capability statistics are wonderful things. These statistics tell you how well your process is meeting the specifications that you have. But there are so many capability statistics that it's worth taking some time to understand how they’re useful together.
能力統計是好事，這些統計可以告訴你：過程是如何符合已有的規範，不過，確實有太多的能力統計，值得我們去花費一些時間去理解這些統計是如何一道起作用的。
Two capability statistics that are hard to keep straight are Cp and Cpk. Their names are different by only a single letter. A single letter that, by the way, doesn’t really explain anything about how these two statistics are different.
在諸多的能力統計中，想要去搞清楚Cp與Cpk確實比較困難，Cp與Cpk在字面上只是有無字母k。順便說一句，單獨的一個字母k，並不能真正的解釋Cp與Cpk有任何不同。
Definition of Cp
Cp的定義
The equation for Cp is often written ET / NT. ET stands for Engineering Tolerance, which is the width between the specification limits. NT stands for Natural Tolerance, which is the width that should contain almost all of the data from the process. Traditionally, NT is 6 times the standard deviation.
Cp，通常被寫作ET或者NT。ET，代表：工程公差，其指代的是規格界限範圍。NT，代表：自然允差，其應該包含過程中的幾乎所有的數據。一般而言，自然允差相當於標準差的6倍。
We often describe Cp as the capability the process could achieve if the process was perfectly centered between the specification limits.
通常情況下，如果過程能夠很好地處在在規格界限之間，我們會將Cp描述為過程可以達到的能力。
Definition of Cpk
Cpk的定義
The equation for Cpk is more complicated: [minimum(mean - LSL, USL - mean)] / (0.5*NT). LSL stands for Lower Specification Limit and USL stands for Upper Specification Limit.
Cpk要更加複雜一點，其計算公式是：[minimum(LSL平均值, USL平均值)] / (0.5*NT)。LSL意思是下限，而USL指的是上限。
Often we describe Cpk as the capability the process is achieving whether or not the mean is centered between the specification limits.
我們通常將Cpk描述為：過程能力是否達到平均值集中在規格界限之內。
Cp and Cpk
Cp與Cpk
The two statistics have a lot in common. The smaller the standard deviation, the greater both statistics are. In fact, under the right conditions, Cp and Cpk have exactly the same value.
Cp與Cpk有很多共同點，標準差越小，兩者就越大。實際上，在一定的條件下，Cp與Cpk值會是相同的。
Here's some data about the volume of ethanol in E85 fuel, which I've manipulated so that Cp and Cpk are the same. Minitab's capability analysis output shows both statistics together.
這是一些關於E85燃料乙醇的容積的數據，我已經操作讓Cp和CPK兩者數值相同。Minitab的能力分析結果將Cp和CPK兩者統計在了一起。
Process capability, shifted to show equal Cp and Cpk.
過程能力轉換顯示Cp與Cpk兩者相同。
The specification limits here are 68 and 83, which are the limits that were in place for the volume of ethanol in E85 fuel in 2010. Halfway in between the specification limits is 75.5. The sample mean of this data is also 75.5 When the sample mean is halfway between the specification limits, Cp and Cpk are the same.
在以上圖表中，規格界限是68和83，這是在2010年，E85燃料乙醇的容積限制。規格界限的中值是75.5，而該數據的樣本均值也為75.5時，樣本均值是規格界限的中值，此刻的Cp和Cpk是相同的。
If the only goal is to have the greatest number of units inside of the specification limits, having the process centered between the specification limits is great. But for some products, other goals compete with having the greatest number of units inside of the specification limits. In the case of the volume of ethanol in E85 fuel, high ethanol content should reduce the use of non-renewable fuels and reduce certain emissions. Plus, most people who buy E85 fuel think that they're buying fuel with volumes of ethanol close to 85.
如果唯一的目標是讓大量的單位數量處於規格界限之內，那麼，讓過程集中處於規格界限之內是很好的。而對於某些產品而言，其他目標與在規格界限內擁有大量的單位數量進行角逐。就E85燃料乙醇的容積而言，高乙醇含量應該能夠減少不可再生燃料的使用，並且可以減少一定的排放。另外，絕大多數購買E85燃料的人認為他們購買的燃料乙醇的容積接近85。
Here’s some real data collected in 2010 by the National Renewable Energy Laboratory on the ethanol volume in E85 fuel. (I』ve removed one outlier where the supplier minimized the volume of ethanol.)
以下這些數據，是在2010年由（美國）國家可再生能源實驗室在E85燃料中乙醇的容積方面收集到的一些真實的數據。
Process Capability of Ethanol Volum--real data
乙醇容積的過程能力-真實數據
The sample mean is about 80.5. This mean is about 5 units higher than the midpoint of the specification limits. The further the sample mean is from the center of the specification limits, the lower Cpk becomes. Also, the further the mean moves away from the middle of the specification limits, the more different Cp and Cpk become.
以上圖表中，樣本均值大約為80.5，該均值大約比規格界限的中點高出5。源於規格界限中心的樣本均值越大， Cpk越低。同時，則進一步意味著：樣本均值越是遠離規格界限， Cp和Cpk則會越發不同。
So this is how Cp and Cpk work together: When the values are similar, the mean is close to the middle of the specification limits. When Cp is greater than Cpk, the mean is nearer to one specification limit or the other. Once you understand your process, you can make a good decision about how to prioritize your process improvement efforts.
這就是Cp和Cpk是如何攜手並進的：當數值相近時，就意味著接近規格界限的中間。當Cp大於CPK時，意味著均值接近一個界限或另一個。一旦了解過程，就可以在如何優先處理過程改進上做出一個不錯的決定。
【特別提醒】
PLY外語學習-用專注的態度，分享職業英文！
識別以下二維碼，關注PLY外語學習！同時，你也會得到自動答覆如何加入公號旗下英文交流群，謝謝！
讀完本文，記得分享出去！