Standard Folio vs Diagram QCP anomoly

[Robbie]

I've started using the diagram function in Weibull++ which I'm finding really useful. I have come across one issue that I don't understand. If I have a data set analysed in a standard folio, I can use the the conditional calculation in the QCP to calculate conditional probability of failure with upper and lower values e.g. using a 2S @ 0.8. If I then select the data set as part of a block diagram and analyse it on its own, the conditional probability of failure is the same, but the upper value is much greater and the lower value much less - as far as I can see I'm using the same analysis method.

Can anyone explain why the results should vary so much, when they should be the same (I think)

[David]

Hi Robbie,

The reason for the difference is that calculations in the diagram have to apply to systems. For a given data set the calculations are straightforward, but when considering a system they have to be modified. Additional information on the confidence bound calculations when using the diagram can be found at http://www.weibull.com/LifeDataWeb/competing_failure_modes.htm#cb. The results from the diagram and the data sheet are both correct. However, since a different method is being used based on the given context, different results are returned. I apologize for the confusion. In the next service release of Weibull++ 7 we will have an update so that the conditional reliability confidence bounds from the diagram for a single block will match the results from the data sheet.

I hope this helps.