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Prep notes for ASQ Certified Reliability Engineer exam ISSN 2165-8633
The CRE Preparation Notes series provides you with short practical tutorials on all the elements that make up the ASQ CRE body of knowledge. The articles provide introductory material, basics, how-tos, examples, and practical use guidance for the full range of reliability engineering concepts, terms, tools, and practices.
Keep your knowledge fresh by regularly reviewing topics and tools that make up reliability engineering.
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You will find the most recent tutorials in reverse chronological order below.
Before computers and statistical software, we relied on tables to determine values for common integration problems – the normal distribution in particular. There is no closed form solution for the integral of the normal distribution probability density function, it requires advanced numerical methods to estimate the area under the curve. [Read more…]
There are two basic ways to consider the central limit theorem. First consider a random variable, X, which has a mean, μ, and variance σ2. If we take a random sample from f(X) of size n and calculate the sample mean, X̄, then as n increases the distribution of the sample means, X̄’s approaches a normal distribution with mean, μ, and variance σ2/√n̄. The original data, X, may have any distribution and when n is suitably large the distribution of the averages will approach a normal distribution. [Read more…]
A continuous distribution is useful in many statistical applications such as process capability, control charts, and confidence intervals about point estimates. On occasion time to failure, data may exhibit behavior that a normal distribution models well. [Read more…]
Learn more and register for the CRE Prep course here. (registration is closed at this time)
The course is 16 two hour sessions – a mix of lecture, discussion and Q&A that focused on what you need to know to pass the ASQ CRE exam. Sure, beyond the course you will need to study. This involved practicing sample exam questions, using your references and calculator, and practicing finding answers quickly.
We discuss ways to study along with any issues or ideas you have as you prepare for the exam.
*Note: These online classes will be Archived and you will be able to access them later even if you cannot make it to the Live class session! This means you may watch them again in case you forget something, miss a class, or can’t make it one day. Saturdays may be scheduled with the instructor for extra sessions or make-up classes if needed.
Mondays, July 13 – August 17, 2015 (3:00PM-5:00PM PDT)
Last Session for questions, September 21 and 28, 2015 (3:00PM-5:00PM PDT)
This class ends prior to the semi-annual ASQ Section testing, where everyone may sign-up through their local Section to take their test. Be sure to check out the ASQ website to get the early-bird rate before the end of this course!
Looking forward to seeing you in the course. And, of course, at any time, feel free to ask questions here, or in the CRE Preparation LinkedIn group.
Learn more and register for the CRE Prep course here.
Similar to the Weibull distribution yet with slightly heavier tails. While not as easy to interpret if the data shows early life or wear out features, the lognormal distribution often fits time to repair data accurately.
Transform the data by taking the natural log of each data point. The resulting values tend to be normally distributed if the original data fits a lognormal distribution.
You can use base 10 or base 2 or any base and the results will still tend to be normally distributed. It is common to use natural log, ln(). [Read more…]
A discreet distribution useful when counting events within a time period.
It models rates, such as the number of gophers in your garden, paint scratches on your car, or the number of shopping carts that arrive in the 5 minutes before you in line. Essentially it’s the count of something over a time period or defined area. [Read more…]
The exponential distribution is a model for items with a constant failure rate (which very rarely occurs).
If the chance of failure is the same each hour (or cycle, etc.), including the first hour, 100th hour, and 1 millionth hour or use, then the exponential distribution is suitable.
Many practitioners assume the failure rate either is constant or changes so little as to be essential constant in order to make reliability calculations simpler. [Read more…]
A point estimate of the mean of a population is determined by calculating the mean of a sample drawn from the population. The calculation of the mean is the sum of all sample values divided by the number of values. [Read more…]
The basic measures of central tendency are mean, median, and mode.
Given a collection of data, a common question is about where the data resides. Knowing the center or mid-point or average is a starting point as we consider the data. [Read more…]
It may be possible to pass the CRE exam knowing one formula.
The math elements of the exam may take a bit of time to solve, and knowing reliability statistics well is a good plan heading into the exam. Knowing the exponential distribution reliability function is one that you should memorize. [Read more…]
Preventive Maintenance or PM is a set of inspections and tasks that help prevent equipment from failing. Keeping equipment operating improves plant capacity and throughput. Avoiding unwanted downtime helps the plant avoid unnecessary expenses and lost productivity.
Checking the oil level in your car’s engine and adding more as needed, along with regular oil changes are examples of PMs. [Read more…]
A reliability block diagram is a graphical and mathematical model of the elements of a system permitting the calculation of system reliability given the reliability of the elements.
The model reflects the reliability performance structure including series, parallel, standby and other arrangements of system elements.
Each block in an RBD represents a component or subsystem of the system. The organization and connecting lines represent the reliability structure of the system and may or may not be representative of the system’s functional block diagram. [Read more…]
A simple assumption in many experiments is to assume the variable act independently on the response.
This means when I change the temperature a little in a polymer dryer silo that time to achieve a certain dryness goes down. And, changing the humidity or airflow rate or pressure either do not change or as they change has no impact on the relationship between temperature and drying time. [Read more…]
The purchase price of a product may be the start of the cost of ownership.
Maintenance and repairs may cost many times the initial purchase price. An efficient maintenance process and low-cost spare parts may help minimize cost, yet a design that optimizes the total cost of ownership is the most effective method. [Read more…]
Dr. Nelson created a simple experiment to illustrate the effect of meddling with a stable process. (Dr. Deming did something similar)
While the intent of making small changes to a process may be to make improvements, these perturbations generally increase the variation of the results.
To improve a process requires understanding the source of the variation and often controlled experiments to identify process improvements. [Read more…]
