Your Reliability Engineering Professional Development Site
All articles listed in reverse chronological order.
Last week I posted about the Binomial probability density function, and it is useful when calculating the probability of exactly x successes out of n trials given p probability of success for each trial.
Well, what happens if you want to know the probability of 2 or more successes for example? [Read more…]
There are many cases where the results of an experiment (or trial) are either it works or it doesn’t, pass/fail, success/failure. Only two possible outcomes one of which we define as success the other outcome as failure. The binomial distribution is suitable if the random variable (the set of experimental or trial outcomes) when
Running through a couple of practice CRE exams recently (yeah, I know I should get out more…) found a few formulas kept coming up in the questions. While it is not a complete list of equation you’ll need for the exam, the following five will help in many of the questions. They seem popular maybe because the relate to key concepts in the body of knowledge, or they are easy to use in question creation. I do not know why. [Read more…]
Received a sample problem from someone preparing for the CRE exam saying it was a tricky one.
The hazard rate function for a device is given by
0.001 if t ≤ 10 hours and 0.01 if t > 10 hours
What is the reliability of this device at 12 hours?
I first draw the hazard function [Read more…]
Down to the last week of preparation for the exam on March 2nd. Good luck to all those signed up for that exam date. Time to focus on preparing your notes, organizing your references and doing a final run through of practice exams. [Read more…]
There are rare situations when we would like to estimate the reliability at the lower confidence level after estimating the mean from a sample (often a test result). And, to make even more rare of a situation, we know the population standard deviation. [Read more…]
Since posting the article about the internal policy by Grundfos I’ve had a range of discussions concerning your efforts to eradicate MTBF. Very encouraging!
The comment by Mark on the Linkedin NoMTBF group prompted me to write about this information. Mark posted: [Read more…]
The operating characteristic curve is used to understand lot sampling plan. It graphically provides a relationship between the unknown lot’s defect rate (or total) and the probability of the specific sampling plan to accept the lot. Very good plans discriminate between good and bad lots. Poor plans may accept bad lots or reject good lots to easily. [Read more…]
A skill needed for the CRE exam is the ability to look up probabilities given a z-value using a standard normal table. It’s old school, I know, yet without software, you most likely will have to find a few values in this manner.
A z-value is the number of standard deviations from the mean at least that’s how I think of it. The area under the curve to the right or left of the z-value is then the probability of interest. Of course, I’m talking about the normal distribution probability density function or what we commonly all the ‘bell-shaped’ curve. [Read more…]
Ideally, in every design of every component, the stress – strength relationship looks like this figure. The stress is well below the strength. [Read more…]
RAMS is an annual multi track technical conference, rams.org. This year in Orlando and about 400 or so attended. I was rather busy, having two tutorials, and three papers. See my site for a copy of my slides and papers. It was fun, exhilarating, and very enjoyable. [Read more…]
A few months ago at a IEC Dependability standards meeting, I met Thomas Young Olesen of Grundfos and we talked a little about NoMTBF. He said their company has a polity to not use MTBF. YES! So I asked for permission to post some information about the policy.
One interesting part of their internal site was a MTBF Calculator. [Read more…]
In the analysis of repairable equipment, one may need to make the assumption of constant failure rate. In my opinion, anytime one wants to make the assumption of constant failure rate, it should be validated. [Read more…]
An error that I see on occasion is the use of statistical confidence with a goal or target value. For example, we want 95% reliability at five years with 90% confidence. What does that mean? [Read more…]
Hypothesis testing permits us to compare two groups of items and determine if there is a significant difference or not. There are many types of hypothesis tests depending on the specific question, type of data, and what is or is not known when designing the test. [Read more…]