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Fred Schenkelberg — Thought Leader

Author of <a href="/creprep/">CRE Preparation Notes</a>, <a href="/on-product-reliability/musings">Musings</a>, <a href="/articles/nomtbf/">NoMTBF</a>, <a href="/book-author/fred-schenkelberg/">multiple books & ebooks</a>, co-host on <a href="/series/sor/">Speaking of Reliability</a>, and speaker in the <a href="/series/sor/">Accendo Reliability Webinar Series</a>.


This author's archive lists contributions of articles and episodes.

by Fred Schenkelberg 3 Comments

Two Pumps Problem

Two Pumps Problem

Let’s say we have two identical pumps share a load in parallel. The failure rate for a pump in this mode of operation is 0.0002 failures per hour. If one pump has to carry the full load alone, that pumps failure rate increases to 0.0009 failures per hour.

What is the reliability of the two pump system over a 168 hour week of operation? [Read more…]

Filed Under: Articles, CRE Preparation Notes, Reliability Modeling and Predictions Tagged With: Failure Rate, Reliability block diagrams and models

by Fred Schenkelberg Leave a Comment

Permutations and Combinations

Permutations and Combinations

A foundational element of probability and statistics is counting. How many ways could something occur? A simple example is a pass or fail criteria, thus when evaluating a product there are two possible outcomes. [Read more…]

Filed Under: Articles, CRE Preparation Notes, Probability and Statistics for Reliability Tagged With: Basic Probability Concepts

by Fred Schenkelberg 7 Comments

Hypergeometric Distribution

Hypergeometric Distribution

In those situations where we sample without replacement, meaning the odds change after each sample is drawn, we can use the hypergeometric distribution for modeling. Great, sounds like statistician talk. So, let’s consider a real situation. [Read more…]

Filed Under: Articles, CRE Preparation Notes, Probability and Statistics for Reliability Tagged With: Discrete and continuous probability distributions, Hypergeometric Distribution

by Fred Schenkelberg 2 Comments

Poisson Distribution Calculation

Poisson Distribution Calculation

Let’s say the results of software testing averaged three defects per 10,000 lines of code. The criteria for release is 90% probability of 5 or fewer defects per 10k lines.

If this product ready for release?

The Poisson distribution is appropriate here as it is useful for modeling defects per unit, count per area, or arrivals per hour. If the data, in this case, the defect count per lines of code to be modeled by the Poisson distribution, the probability of an occurrence (defect in this case) has to be proportional to the interval (lines of code in this case). Also, the number of occurrences (defects) per interval must be independent (more on statistical independence in another post). [Read more…]

Filed Under: Articles, CRE Preparation Notes, Probability and Statistics for Reliability Tagged With: CDF, cumulative distribution function, Discrete and continuous probability distributions, Exponential Distribution, PDF, Poisson Distribution, probability density function

by Fred Schenkelberg 17 Comments

Calculating Lognormal Distribution Parameters

Calculating Lognormal Distribution Parameters

The lognormal distribution has two parameters, μ, and σ. These are not the same as mean and standard deviation, which is the subject of another post, yet they do describe the distribution, including the reliability function. [Read more…]

Filed Under: Articles, CRE Preparation Notes, Probability and Statistics for Reliability Tagged With: Discrete and continuous probability distributions, function, Lognormal Distribution, reliability, Standard Deviation

by Fred Schenkelberg 9 Comments

ALT and HALT

ALT and HALT

The other day I got a question about the difference between ALT and HALT. There was some confusion probably because of the similar words in the acronym. ALT is Accelerated Life Test, and HALT is Highly Accelerated Life Test. [Read more…]

Filed Under: Articles, CRE Preparation Notes, Reliability Testing Tagged With: Accelerated life tests, Discovery Testing

by Fred Schenkelberg 7 Comments

Effective FMEA Principles

Effective FMEA Principles

Failure mode and effects analysis, or FMEA, is a tool for the identification and prioritization of possible ways a product or process can fail. The intent is to use that information to make improvements to the product or process.

I think of FMEA (and related processes like FMECA, dFMEA, etc.) as structured brainstorms that provide a means to focus on what’s important. [Read more…]

Filed Under: Articles, CRE Preparation Notes, Reliability in Design and Development Tagged With: FMEA and FMECA

by Fred Schenkelberg Leave a Comment

Reliability Goal

Reliability Goal

The reliability goal is a key element across the entire product lifecycle. From product definition to determining warranty to judging performance, knowing the goal in clear terms sets the stage for a successful product.

Reliability in engineering terms is the probability of satisfactory product performance within a defined environment over a stated duration. [Read more…]

Filed Under: Articles, CRE Preparation Notes, Reliability in Design and Development, Reliability Management Tagged With: Customer Needs Assessment, Establishing specifications, Probability, Product Reliability, warranty

by Fred Schenkelberg 5 Comments

Sequential Sampling by Attributes

Sequential Sampling by Attributes

There a few different ways to sample a lot (or group) of material to determine if it has an acceptably low failure rate (or proportion that are considered ‘bad’). The following is an example of the sequential sampling method, which happens to be rather efficient by generally using the fewest samples for the same risk protection. [Read more…]

Filed Under: Articles, CRE Preparation Notes, Probability and Statistics for Reliability, Reliability Testing Tagged With: Qualification Demonstration Testing, sampling

by Fred Schenkelberg 2 Comments

Reading CDF Plots

Reading CDF Plots

First the Question:

Fred,
Early in the FMEA lecture you worked through a homework problem and you mentioned that a cdf may not be linear (hence the reason for giving three points in a reliability goal).  Can you give an example of two of things you’ve seen with non-linear cdf’s?  I’ve only done limited reliability testing at this point, but everything I’ve done and every example I’ve ever seen have had linear cdf’s.
Thanks,
John

And, my response: [Read more…]

Filed Under: Articles, CRE Preparation Notes, Data Collection and Use, Probability and Statistics for Reliability Tagged With: CDF, cumulative distribution function, plot

by Fred Schenkelberg Leave a Comment

EAM & CMMS Systems, 10 times more data in the system or 10 time less done with the data available?

EAM & CMMS Systems, 10 times more data in the system or 10 time less done with the data available?

10 times more data in the system or 10 time less done with the data available?

EAM & CMMS Systems, 10 times more data in the system or 10 time less done with the data available?.

A nice short article about the problem of data, data, too much data.

Filed Under: Articles, CRE Preparation Notes, Data Collection and Use, Maintainability and Availability Tagged With: Data Management

by Fred Schenkelberg Leave a Comment

More Errata

More Errata

CRE_Errata_9-26-2011_revised

The CRE_Errata_9-26-2011_revised document is in pdf format and contains a couple of typos noted by John Cooper in the Indiana Council CRE prep materials – he has taught the CRE Prep course for Ops A La Carte and is a pretty good teacher and a CRE. [Read more…]

Filed Under: Articles, CRE Prep, CRE Preparation Notes

by Fred Schenkelberg 2 Comments

Series Reliability Question

Series Reliability Question

Recently a colleague sent me a published copy of the first CRE exam. Scanning through the document suggests that a few things have changed and many have not changed at all. I often comment my enjoyment of the reliability engineering profession, as it doesn’t change too fast and even I can keep up. [Read more…]

Filed Under: Articles, CRE Prep, CRE Preparation Notes, Reliability Modeling and Predictions Tagged With: CRE, Series System

by Fred Schenkelberg 3 Comments

RBD and Design Process

RBD and Design Process

A reliability block diagram (RBD) for a product that has no redundancy or complex use profile is often very simple. A series system (reliability wise) implies that any one part or element of the product that fails the entire product fails. One might ask if an RBD is even necessary. [Read more…]

Filed Under: Articles, CRE Preparation Notes, Reliability in Design and Development Tagged With: design, Design Process, Reliability block diagrams and models

by Fred Schenkelberg 2 Comments

Top Tips for Probability Analysis

Top Tips for Probability Analysis

This is provided courtesy of Amanda at

Egerton Consulting Ltd
Visit our website at www.egertonconsulting.co.uk

<They have a great newsletter and worth subscribing. Fred> [Read more…]

Filed Under: Articles, CRE Preparation Notes, Probability and Statistics for Reliability Tagged With: Probability

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