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Fred Schenkelberg


The Relationship Between Reliability Goals and Statistical Confidence

From the desired state to a reflection of sample risk during measurement

Published: Wednesday, October 5, 2016 - 00:00

We establish reliability goals and measure reliability performance. Goals and measures can be related; however, they’re not the same, and neither do they serve the same purpose.

Recently, I’ve seen a few statements that seem to confuse the role of statistical confidence when establishing a goal. Thus, I’d like to relate how I think about the difference between goals and statistical confidence, along with how they are related.

The purpose of a reliability goal

Setting any goal provides tangible direction or a meaningful target for a team. A reliability goal is a balance of:
• What the customer expects
• What is technically possible
• An expression of business objectives

A reliability goal establishes the probability that a function will successfully operate over a specific duration, given a specific use and environment. For example, my smart phone will make and receive calls in Northern California with a 99-percent probability of successful operation over two years.

The purpose of a reliability goal is to define to the engineering team, marketing team, and customers the item’s expected or desired reliability performance. It’s not the actual reliability performance; it’s what we would like to happen. It’s a goal, an objective, a target, and its purpose is to provide guidance, alignment, and requirements for creating a new product or system.

The purpose of statistical confidence

Statistical confidence is a statement of the acceptable risk we are willing to accept. The risk is based on the ability of a specific sample to accurately represent the entire population.

The chance that a single sample’s reliability performance will be the same as the population’s average reliability performance is very low. As we measure more samples from the population, the ability of the sample to estimate the population’s performance increases. There’s some statistical law about this phenomenon.

Sometimes a policy is set concerning the amount of risk we are willing to accept when using a sample to create an estimate. Other times we determine the statistical confidence based on how many samples we have evaluated.

The purpose of statistical confidence is to manage the risk that the sample will provide misleading information.

The goal is often to minimize sample-size-related risk within the constraints that exist in limiting sample sizes.

The relationship between goals and confidence

Reliability goals are a desired state of performance. Statistical confidence reflects the risk that a sample-based result is close to being true.

Stating a reliability goal that includes a confidence in effect defines the desired sample size to measure the reliability performance. That is not a bad relationship. Yet stating we want a reliability goal of 90-percent reliability over two years with 90-percent confidence, is not, in my mind, a goal.

We would like the actual reliability of the item to have a 90-percent chance of surviving two years. We may want to minimize sample risk by measuring the reliability performance with a 90-percent confidence.

The estimate from a sample of the actual reliability performance will always be an estimate. The statistical confidence provides a figure of risk indicating that the results we are viewing actually reflect the real and unknown reliability performance.

If we use a one-sided lower bound for the confidence, we are looking for evidence that the reliability is at least above the lower-bound value. We could set the threshold for the lower bound at the goal value. With 90-percent confidence, we are accepting the risk that the actual population reliability is less than 90 percent, and that the sample results will end with a value higher than the goal; thus, we believe the product’s reliability performance is better than actual.

The relationship between a goal and confidence is that they both provide information about the unknown actual reliability performance. A goal is the desired state; confidence is a reflection of sample risk during measurement.

Let me know if this makes sense, or if there is a better way to describe the relationship between goals and confidence.


About The Author

Fred Schenkelberg’s picture

Fred Schenkelberg

Fred Schenkelberg is an experienced reliability engineering and management consultant with his firm FMS Reliability. His passion is working with teams to create cost-effective reliability programs that solve problems, create durable and reliable products, increase customer satisfaction, and reduce warranty costs. Schenkelberg is developing the site Accendo Reliability, which provides you access to materials that focus on improving your ability to be an effective and influential reliability professional.