Articles

Find all articles across all article series listed in reverse chronological order.

Z Value

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…]

by Fred Schenkelberg 19 Comments

Stress Strength Normal Assumption

Ideally, in every design of every component, the stress – strength relationship looks like this figure. The stress is well below the strength. [Read more…]

by Fred Schenkelberg 7 Comments

Mann Reverse Arrangement Test

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…]

by Fred Schenkelberg 4 Comments

Statistical Confidence

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…]

by nomtbf Leave a Comment

MTBF and preventative maintenance

I find the world of maintenance a very odd place to find MTBF. While it is possible, that a set of equipment or a machine may actually have a constant failure rate it is the exception rather than all that common. Assuming a constant failure rate doesn’t make it so. [Read more…]

by Fred Schenkelberg Leave a Comment

Hypothesis Test Sample Size

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…]

by Fred Schenkelberg 1 Comment

Reliability Centered Maintenance

The document by Nowlan and Heap

Part of keeping equipment operational is affecting repairs appropriately. Sometimes it makes sense to replace an element of some equipment before it fails, i.e. automobile tires as the tread wears. Sometimes is doesn’t make sense to repair a functioning unit as the result would be a system that is less reliable then before. [Read more…]

by Fred Schenkelberg Leave a Comment

Emerging Technology Reliability Considerations

As we continue to find and improve ways to master our environment, accomplish tasks, communicate, move, heal, etc. we continue to discover new materials, processes, and products. Emerging technologies often promise

lower costs
new functions
lighter weight
easier to use

They also may last longer.

As a minimum, they should last long as current solutions or longer or provide another compelling reason to use the new technology. [Read more…]

by Fred Schenkelberg Leave a Comment

2012 in Review

The WordPress.com stats helper monkeys prepared a 2012 annual report for this blog.

Here’s an excerpt:

600 people reached the top of Mt. Everest in 2012. This blog got about 11,000 views in 2012. If every person who reached the top of Mt. Everest viewed this blog, it would have taken 18 years to get that many views.

Click here to see the complete report.

by Fred Schenkelberg Leave a Comment

2012 ASQ Salary Survey is out

Once again those with a CRE are at or near the top of the salaries reported in this survey. See more at http://www.qualityprogress.com/salarysurvey

by Fred Schenkelberg 16 Comments

CRE Exam Preparation

I never did like taking final exams in college as I viewed them as four hours I would rather spend doing something (anything) else. An exam is a check of what you know about a topic or subject. We prepared for those exams by reading, studying, practicing working problems (doing homework) and trying to understand the key concepts, trends, themes, etc. Some of my classmates thought it was about how much they could stuff into short term memory, others though sleep deprivation had something to do with preparing for an exam. [Read more…]

by Fred Schenkelberg 5 Comments

M and A Sample Questions

Just a couple of sample questions from deep in the body of knowledge this week.

Which of the following can be evaluated with dye penetrant methods? [Read more…]

by Fred Schenkelberg 5 Comments

Question a Day Idea

Hi all,

I’m asking for feedback here. I’m thinking about building a subscription service that sends the subscriber a CRE preparation question a day (maybe adjustable by subscriber … not sure how to do this yet). [Read more…]

by Fred Schenkelberg Leave a Comment

Why HALT is a methodology, not equipment

Kirk Gray, Accelerated Reliability Solutions, L.L.C.

It is easy to understand why the term HALT (Highly Accelerated Life Test) is so tightly couple to the equipment called “HALT chambers” systems. Many do not think they can do HALT processes without a “HALT Chamber”. Many know that Dr. Gregg Hobbs, who coined the term HALT and also HASS (Highly Accelerated Stress Screens), spent much of his life promoting the techniques and was also the founder of two “HALT/HASS” environmental chamber companies. [Read more…]

by Fred Schenkelberg 2 Comments

Sample Size Problem

Let’s work a sample size problem.

A random sample size, n, is to be taken from a large population having a standard deviation of 1″. The sample size is to be determined so that there well be a 0.05 risk probability of exceeding a 0.1″ tolerance error in using the sample mean to estimate μ. Which of the following values is nearest the required sample size?

a. 42

b. 106

c. 203

d. 384 [Read more…]