Mood’s Median Test
This nonparametric hypothesis test tests the equality of population medians. While not as powerful as the Kruskal-Wallis Test, it is useful for smaller sample sizes, when there are a few outliers or errors in the data as it focuses only on the median value.
One assumption to check concerns the similarity or equality of the underlying variances (or shapes) of the different…
Information for Maintenance Planning
One of the considerations when planning maintenance is the timing of replacement of parts of the system. In some cases it makes sense to wait for the item to fail, sometimes, it makes sense to replace on a interval or set amount of operating time, and yet in others we can measure indicators that a replacement is necessary.
I order to make these decisions we…
Expectation and Moment Generating Functions
In statistics and reliability we use distributions to describe time to failure patterns. The four functions commonly used in reliability engineering include
- The probability density function
- The cumulative distribution function
- The reliability function
- The hazard function
We often use terms like, mean, variance, skewness, and kurtosis to describe…
Benefits of Reliability Engineering
We see the terms ‘reliable’ and ‘reliability’ in daily advertising, business names and in casual conversations on a regular basis. Reliability has meaning and importance in our society. Product and brand reputations are made or broken by their product reliability performance.
Reliability engineering or the work to minimize failures, improve maintenance…
Censored Data and CDF Plotting Points
Reliability testing and data collection is messy business. We rarely receive perfect data where all units involved have a precise time to failure record. Sometimes we do not precise time to failure information or some of the units are still operating.
The data is censored. There are a few common types of censoring each of which have statistical techniques to…
4 Electronic Nondestructive Evaluations
It is possible to use the circuit design or the circuit itself to evaluate electronics. This ranges from checking is the system is wired correctly (the right parts are present and attached properly) to a complex evaluation including the proper functioning of firmware and attached software.
Beyond visual inspection, we need a way to evaluate the microscopic…
8 Nondestructive Evaluation Techniques
There are times when we need to evaluate a product to determine is it assembled correctly or when looking for clues concerning a failure mechanism. Simple visual inspection may be sufficient and there are times when we need more information or detail.
A great first step is the use of an appropriate nondestructive evaluation (NDE) method. Done correctly we…
8 factors of design for maintainability
The first time I changed the oil filter of my car (first car in high school) I smashed my knuckles against an grimy block of metal. More than once. It may have been my lack of experience or improper tools. Or, it may have been a rather poor design
Watching the Indy 500, especially the pit stops, I quickly realized there has to be something different about…
Considerations for Reliability Testing
Reliability testing to determine what will fail or when will failures occur is expensive. Organizations invest in the development of a product and attempt through the design process to create a product that is reliable.
The design process has many unknowns though. This includes uncertainties about materials, design margins, use environments, loads, and aging…