## ISO 26262とPMHFのコンセプト(続き)

The PMHF and all the related failure rate calculations in a product development team are often handled by a reliability engineer and even though the standard uses the metrics reflecting the probability of failure (or unreliability in reliability engineer’s terms), some of the approaches and metrics used by ISO 26262 are not that of reliability engineering practice. Therefore, the objective of this paper is to explain some of the metrics and calculations suggested by the Functional Safety standard in reliability engineering terms in order to make their application easier.

「故障の確率（信頼性技術者の用語では不信頼度）」という表現は、ISO 26262的には誤りです。故障の確率や不信頼度(これらは修理を含まない)ではなく修理を考慮した不稼働度が正しい表現です。次の章で説明しますが、これについてはほとんどの論文が同じ誤りを犯しています。その理由は、規格に数学的な説明が無いためであり、これはPMHF式を自ら導出して初めて理解されることです。

## PMHFと基本的な信頼性計算

In order to link the PMHF and reliability terminology, certain basics of reliability calculations need to be briefly covered here which can be found in multiple sources (see, for example, [3]). If the random failures in the field can be modeled by a statistical distribution with the probability density function (pdf) f(t), then the cumulative distribution function (CDF) F(t) representing the probability of failure at the time t or unreliability can be calculated as:

PMHF と信頼性の用語を結びつけるために、信頼性計算のある種の基礎をここで簡単に説明する必要があります。現場でのランダムな故障が確率密度関数(pdf) f(t)を持つ統計分布でモデル化できるならば、時刻 t での故障の確率または信頼性の低下を表す累積分布関数(CDF) F(t)は次のように計算できます。

$\dagger$: Kleyner, A. and Knoell, R., “Calculating Probability Metric for Random Hardware Failures (PMHF) in the New Version of ISO 26262 Functional Safety - Methodology and Case Studies,” SAE Technical Paper 2018-01-0793, 2018

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