What is Weibull++ doing in Mixed FM Analysis?

[Steve]

..that is, what exactly to the proportions mean, and where do they start and end. I had a data set that appeared to have 3 distinct FM's, but the individual slopes did not match what the software was finding. I realize you can't just break the data up cleanly where plot seem to have "bends", but, I don't know what Weibull++ is doing, so it makes me leary of using it.

[Arai.M]

Hello,
Our reference for Mixed weibull can be found at http://www.weibull.com/LifeDataWeb/the_mixed_weibull_distribution.htm
specifically, you can take a look at http://www.weibull.com/LifeDataWeb/mixed_weibull_parameter_estimation.htm
where the equations that are being solved are shown. Note that there is no details on the numerical method used, namely Levenberg-Marquardt algorithm (non-linear regression) but my guess is that that's not what you are asking. As to why the line doesn't follow the points, you need to keep in mind that the points are plotted according to median ranks while the solution makes no use of median ranks so this is not completely representative. If you want to assess fitness you are better off using statistical methods rather than a graphical evaluation.
Arai