Use the Right Fit

I’ve often railed on and on about the inappropriate use of MTBF over Reliability. The often cited rationale is, “it is simpler”. And, I agree, making simplifications is often necessary for any engineering analysis.

It goes too far when there isn’t any reason to knowingly simply when the results are misleading, inaccurate or simply wrong. The cost of making a poor decision based on faulty analysis is inexcusable.

Using even just a better fit makes a big difference. I suggest that instead of using total time divided by total failures – use the time to failure information, which you probably already have available for the analysis.

Recently, I’ve been working with a few clients with this simple recommendation. Most opt for a data analysis and plotting package that is relatively inexpensive and very easy to use – Reliasoft’s Weibull++ is one such package.

Another path open to a bit more series students is the freely available GNU license software package called R. You can learn more and download the software at

http://cran.r-project.org/

While this is primarily a statistics programing/scripting language – it is fully functional for reliability statistics, too.

The Weibulltoolkit package

This morning I ran across a package that creates Weibull plots on the appropriate Weibull scales (just like Weibull plotting graph paper). It also permits all the graphical and analysis control of R. Very powerful and flexible.

I will say the learning curve is a bit steep. It is after all a programing language. And, there are plenty of articles, books, documents, websites, courses, etc. available to get you up to speed.

I created the plot above with a single line of code

plot.wba(Surv(wbparams.to.ft(5,2,2000)),col=”red”)

The R function, plot.wba, is a slightly modified version of the command plot.wb contained within the weibulltoolkit package. You can learn more about the toolkit from the paper by Jurgen Symynck and Filip De Bal, titled “Weibull Analysis Using R, in a Nutshell.” You can find the paper at

http://mechanics.kahosl.be/fatimat/images/papers-books/paper-weibull_analysis_using_r_in_a_nutshell.pdf

Which ever software you use – avoiding the simplest route is worth the effort.