A typical use for a demonstration test plan is to show the improvement of a redesigned system. For this purpose, you can frequently use a historical value to estimate the shape (Weibull distribution ) or scale (other distributions) parameter. The historical value is frequently a good estimate because the redesign does not affect this parameter. However, if your estimate of the parameter is wrong, then the results of the demonstration plan can be inaccurate. The sample size or testing time that Minitab calculates could be larger or smaller than necessary.
You should consider repeating the demonstration test plan analysis in Minitab using a range of likely values for the assumed parameter. The results from the different analyses can help you to determine how sensitive the sample size or testing time is to the assumed parameter.