Issue 33, November 2003

What are the dates and locations for ReliaSoft's 2004 public reliability training seminars?

The following table presents the dates and locations for ReliaSoft's public reliability training seminars in 2004. If your schedule conflicts with these public training seminars or if your organization requires customized training, please contact ReliaSoft to discuss on-site seminar options.

When using the Weibull distribution, why does it seem that the mean increases as the value of beta decreases (0 < beta < 1) for a constant value of eta?

Actually, this is one of the reasons why the mean is such a misleading metric. As mentioned, the mean increases as beta decreases (for the same eta and for 0 < beta < 1). However, if you calculate the probability at the mean, you will see that there is a higher probability of occurrence at the lower beta. In reliability engineering, this means that a higher portion of the population will fail by the mean life. It also means that a higher percentage will fail initially, but there is also a small percentage that will survive a long period of time (compared to the initial failures). This portion is what is pushing the mean to the right. In other words, you have a much higher spread (or standard deviation). The smaller the beta, the higher the spread. Relating to the pdf, even though the curve leans to the y-axis as beta goes to zero, you will notice that it also extends further on the x-axis (i.e. approaches zero less rapidly).

Why doesn't BlockSim allow for the rolling up of system reliability uncertainty using confidence limits on the individual component parameters?

We have looked into adding this feature to BlockSim, but there are two issues that need to be addressed:

1. All the blocks in the diagram must have the associated uncertainty on the parameters included in their properties. Even if one block does not have these values then the analysis should not be performed.
2. As the number of blocks in the diagram increases, the bounds on reliability become unrealistic (i.e. between 0 and 1).

We are continuing to look into this and we may include this feature at some point, regardless of these drawbacks.

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