Technical Appendix
前話で触れられた工程能力指数（CpおよびCpk）について、簡単に補足します。
物語の中では会話の一部として登場しましたが、数値そのものの意味を整理しておくことで、背景にある構造が見えやすくなるかもしれません。
On Process Capability
The conversation in the office had sounded reasonable.
Numbers were mentioned.
Targets were cited.
Capability had “declined.”
In manufacturing, Process Capability Indices—Cp and Cpk—are used to describe how consistently a process operates within specified limits.
Cp measures the spread of variation.
Cpk considers both variation and alignment with the target.
A value of 1.33 is often regarded as satisfactory.
Below that, concern begins to rise.
But a number does not explain itself.
A lower Cpk may reflect worn tools, subtle material variation, accumulated drift, or shifts that no single operator can immediately correct.
It is a description of variation—not a verdict.
Yet, once tied to evaluation, promotion, or trust, the number acquires weight beyond its formula.
For Sakurako, Cpk is not an accusation.
It is a reference point repeatedly invoked in conversation.
The metric itself remains neutral.
How it is interpreted, and by whom, is not always neutral.
Statistics were developed to distinguish systemic causes from individual error.
Whether that distinction is preserved depends not on the equation, but on the structure surrounding it.
今回は工程能力指数についての補足でした。
数式や定義そのものよりも、それがどのような場面で使われるのかに注目していただければと思います。
次回は、本編の日本語訳を掲載予定です。
ご感想やご意見がありましたら、ぜひお聞かせください。今後の執筆の参考になります。
