Distribution distortion in the statistical analysis of strength data when using multiple specimen batches

•The strength of Zerodur®, as machined and etched is best represented by a normal distribution with large variance.•Thus, a different design approach might be used for structural design with this flaw population.•The appearance of a 3-parameter Weibull distribution results from mixing of components of variation.•Although a 3-parameter Weibull distribution might be reasonably fitted, the size effect is missing, and the fixity of the threshold is questionable.•Caution is advised when assuming a 3-parameter Weibull distribution and associated threshold solely based on distribution shape.