Sample size considerations
Detecting a change or difference is often the aim of an experiment or set of measurements. We want to learn which vendor, process, or design provides a better result.
When we use a sample to estimate a statistic for a population we take the risk that the sample provides values that are not representative of the population. For example, if we use a professional…
The Mann-Whitney U Test
The U test permits the comparison of two samples to determine if they came from the same population or not. This non-parametric test can use ordinal data, meaning it is in some rank order without containing information about relative distances between ranks.
For example if we ask people to score brand preference on a 5 point scale, we could compare brands. Or we can…
Laplace’s trend test (Centroid Test) for a Single System
A test to determine is the homogeneous Poisson model (HPP) is applicable given the data from an individual system is subject to the non homogeneous Poisson model (NHPP).
This is an alternative test to using the Kendall-Mann Reverse Arrangement Test.
Use the arrival times of failures (not time between arrivals or inter arrival times) for the…
When something fails, what should we do?
A natural question when something fails is
Why did it fail?
The answer is not always obvious or easy to sort out.
One of my favorite examples was on a circuit board that had a small burn mark where a component exploded off the board. The customer didn’t notice that missing part, our engineering team did that.
The customer noticed the features they wanted…
3 Steps Nationally recognized testing laboratories use to evaluate product safety
With any product development there is a risk the features (benefit) come along with inherent dangers (risk). For example a desk top computer includes the need for electrical power. Done improperly a person exposed to wall current and voltage could be seriously harmed. While unlikely the risk exists.
One of reasons…
Reliability Growth Testing
One approach for reliability improvement is to find the weaknesses or faults, then fix them.
This is best done before shipping to the customer.
You may hear this called the Test, Analyze, and Fix Method (TAAF). Or, you may have heard of it called a Reliability Growth Test program (RGT). Either way, the essence is evaluating prototypes to find specific faults.
Testing a new design is expensive and takes time to accomplish. During the design process engineers make assumptions across many decisions. Unknown interactions and unanticipated events may lead to product failure.
Finding design weaknesses early in the design process permits the team to address and correct problems.
Discovering the weaknesses is one way to provide evidence that…
This might be easier to read printed out. Or double click on image and enlarge on your screen.
How to read a standard normal table
One of the most basic skills needed for the CRE exam is the ability to correctly read the Standard…