Weibull++ Standard Folio Data 4 Subpop-Mixed Weibull

Reliability Web Notes
Weibull Folio
Life Data Analysis
The mixed Weibull distribution (also known as a multimodal Weibull) is used to model data that do not fall on a straight line on a Weibull probability plot. Data of this type, particularly if the data points follow an S-shape on the probability plot, may be indicative of more than one failure mode at work in the population of failure times.
[math]\displaystyle{ {{f}_{1,...,S}(T)} }[/math] is given by [math]\displaystyle{ \underset{i=1}{\overset{S}{\mathop \sum }}\,\frac{{{N}_{i}}{{\beta }_{i}}}{N{{\eta }_{i}}}{{\left( \frac{T}{{{\eta }_{i}}} \right)}^{{{\beta }_{i}}-1}}{{e}^{-{{(\tfrac{T}{{{\eta }_{i}}})}^{{{\beta }_{i}}}}}} }[/math] where S=2, S=3, and S=4 for 2, 3 and 4 subpopulations respectively.
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